Cycling + Water Changes

[Billiemay]

Could someone please tell me how often i should be doing waterchanges in my 27L tank, its got a 25w heater, real plants, no fish yet will be used for a betta. 

 

[fluttermoth]

A good average to aim for, and what most of us here seem to do, is 50% a week but, when you get your fish, it's a good idea to monitor your nitrate levels.
 
Water changes are about more than reducing nitrate, but that's a good indicator if your water changes are big enough or often enough.
 
It's best to do enough water changes to stop the nitrate from getting to more than 20ppm above what's in your tap water

 

[eaglesaquarium]

+1 to fluttermoth's comment.
 
 
How many plants?  And are you planning to do a fish-in or fishless cycle?

 

[Billiemay]

I have 3 plants and I'm doing a fish less cycle

 

[eaglesaquarium]

If the betta is the only thing in the tank, then 25% weekly should be fine... But you'll need to play that one once you start collecting some data on the tank.  Nitrates at the beginning of the week, nitrates at the end, overall health and demeanor of the betta, etc.