The answers, such as they are, are probably available only at Kew with the Muster Books and Pay Lists (that's one document produced every quarter, usually). See TNA Guide on Militia
The problem is that these books are usually very thin on genealogical data so it may not be possible to distinguish 2 people with the same name. Tracking how long someone served can only be done by following these books forward and back to see when someone appears and disappears.
The next issue is that 11th British Militia isn't a name of a unit. But given the Scottish context it should probably read 11th North British Militia (Regiment). The only way is to work on that basis. I have no idea quite where the 11th was based - my first check suggests that it wasn't one of the originals in Scotland - but the MB&PLs do indicate where a unit was.
Whether any local military museum in Scotland has any more data, I've no idea. Personnel papers for individual soldiers in this era only survive for those serving long enough to get a pension.
To add a bit more detail:
The original poster has identified a page on the Forfar & Kincardine Militia that says that the Forfarshire, or 8th North British Militia was raised in 1798, that it became the Forfarshire and Kincardine Militia in 1802, was disembodied but then re-embodied in 1803 as the 11th or Forfar(shire) and Kincardine Militia.
Assuming that page to be correct, and I have no reason to doubt it, then the relevant Muster Books & Pay Lists are those for the Forfar & Kincardine and are at Kew (using that version of the title so far as I can see), starting with:
Reference: WO 13/778
Description: Forfar & Kincardine
Date: 1798
and going through to
Reference: WO 13/795
Description: Forfar & Kincardine
Date: 1815
These are 1 piece per annum. The sequence then goes through to WO 13/803 but in multiple years per piece. However, while you should be able to find Charles Ormond in that lot (if the register entry is correct) and identify which year he joined and when he left (possibly even the exact dates given that these are Muster Books & Pay Lists), if there are 2 Charles Ormonds you may not be able to identify which is which.
As an example of the issues, I have looked at MB&PLs for the Volunteers of South Cheshire during the same era - I see names that match my relatives but nothing to identify whether the guy is my relative, not even which town or village in South Cheshire the soldier and his company come from.