We get asked this question a lot and there is no such fixed timeframe as such. It all depends on multiple factors. But I would like to share the following based on our experience :
Most good venues get booked in advance for popular wedding dates, so if you are fixed on the wedding dates, and if you like the venue, and it is available, it makes sense to book it.
But do keep in mind, most wedding venues have some sort of penalty for cancellation & even re-scheduling, therefore if there is dependency on issues like Visa, vacation approval from employer, travel plans of bride & groom, or close family, health of close family members, etc it makes sense to wait to get a green confirmation before going ahead.
3-6 months is a good timeframe to look for most venues in Bangalore.
If you are looking at getting married during popular marriage seasons in India, it makes sense to have multiple dates handy, because your favourite venue might not be available. There is always this "the best amongst the best dates" kind of logic from the family astrologer, and it takes a lot of effort to move that mountain :-)
Having said this, venues will be available even in short notices of less than 3-months. There might be cancellations, or just simply available lucky dates. Also it helps if you are flexible on the duration or sequence of events because then you can fit into odd spots.