Upvote:2
It's largely up to the consulate processing your application. The Handbook for the processing of visa applications recommends issuing a visa valid for the duration of the trip (as documented in the application) plus a grace period of fifteen days (to make things simpler, in case you get delayed). But it even happens that consulates issue visas that are too short for the intended trip.
Since you are presumably a resident in the UK, you might however get lucky and obtain a six-month multiple entry visa without requesting it. If you have been to the Schengen area before and justify a need to travel frequently, you could even get a visa valid for up to five years.
No matter the validity of the visa, you will not be allowed to stay more than 90 days in the area in any 180-day period.