I've been researching my family history, and I hit a roadblock. The oldest records I can find are from German church records in the early eighteenth century. I can find nothing older. Is this a reasonable amount for German ancestry?
There are all kinds of special cases, but a rough answer is possible. In Western Europe, the surviving consistently kept church records generally start around the 17th century, but that does vary from country to country, province to province and finally, from church to church. For the Netherlands, neighbours to Germany, 1650 is a reasonable but rough number to keep in mind.
Where there are no church records any more, you might find genealogical fragments in legal records of the towns, but few of these have been transcribed or indexed yet. A link to aristocracy may add a few hundreds years to one particular branch, with records of variable quality, and could include a line back to Carolus the Great (Charlemagne), born in the 8th century. The reliability of the records for early aristocracy is a specialised topic.
To get back to the crux of question, should you be satisfied with tracing a particular line of Western ancestry back to early 18th century, the general answer, apart from heh, that is pretty good already, is probably not, but it depends.
You might be able to trace back into the 17th century, and may even find a few names of people born in the 16th. However, there are exceptions, and records that were once there may have been lost in burned down churches or bombed town halls. They might also survive, but after hundreds of years of European history, not be where you expect to find to them.
For the most definitive answer for your line, seek out the local archives, genealogy and history societies, most of which will already have a web site providing general information about the history of the area, and the records available for genealogical research.
First of all, you should be extremely happy to find your family records back to the 18th century.
Second, there are digitized microfilm records from Germany going back to the 1300's on the Familysearch.com website. From the homepage choose "Continental Europe" under the Browse by Location heading on the lower left. Then choose Germany and see what records are digitized.
That's the quickest way to see what is available to you at your computer.
Since the Council of Trent (Concilium Tridentinum - 1545-1563), it has been mandatory for all parsons in the Catholic Church to record all events (christening, marriage and burial). Before this Council not all parishes kept such records. It took 'a few years' (in the lands of Czech crown almost a whole century, in Western Europe less) to apply this rule.
Another significant factor in European history was the 30 Years' War, which destroyed a large part of continental Europe. (Read: big part of records that had been kept were burned.) Even though not all regions were directly affected by battles, a lot of them suffered from the movement of armies or military corps.
General question - general answer ;)
I have traced my own roots back about as far. Mid-19th century Ireland, through the help of a few records that have been freely transcribed on http://www.irishgenealogy.ie/index.html. Yet through Ancestry.com I was able to trace my girlfriend's Cady roots back to the late 16th century.
Many Irish records were destroyed during the uprising in 1916. As a result, it is local parish records that need to be indexed and transcribed yet, that will enable those with Irish roots to explore their family's more distant past.
There have been a lot of political changes in what is now Germany. As a result, those records may have been lost at one level but still exist somewhere else. But there's no doubt that roots (in general) can be traced much further back than you have had the opportunity to go.
Another problem in older records (if not all records) is a lack of detail in an entry. Example: a christening record that names the father but not the mother.
As result, the researcher must look at the whole set of records in a location to distinguish families with similar names. Gaps in any sequence may make it impossible to progress to the next generation.
A couple of years back I ended a 3-year search in a town in Italy where church and land transfer records date back to the last quarter of the 16th century. It was a very complicated undertaking that was used as a test case during the government's certification of the local church archive. I can tell you I was extremely lucky to have presented information going back to 1857 and then having a PhD of Historical Records trace the line of one family name back to a will in 1572 in local notary records. Only the fact that Italian church records are very complete in details such as maiden names and the names of parents of brides and grooms allowed for this. Of course, it was important that all never left the town and were involved in the same trade, stone masonery. If you have all these details present and the records are complete, then a search back 500 years to the furthest edge of Council of Trento church records is possible in Catholic Europe. The fact, that a family was well-to-do enough to be involved with wills and the transfer of land is a great assist in this attempt. Also, be prepared to spend quite a bit of money. I have over 200 pages of photographed and translated entries from old Italian and Latin covering the search line back from a marriage in 1872 to a will written in 1572. The cost was over $6500. US dollars and involved three trips to Italy. The great reward is in the fact that it is all real and I wouldn't have it any other way. Many people involved in genealogy are out to deceive themselves in the pursuit of their past. The biggest hurdle is the fact that your're almost certain to be denied access to search valuable antique records. As I said, I was luck to have been in the right place at the right time.