How often to put your child on the toilet?

My daughter has just turned 3 & hasn't really shown much interest in toilet training. We've been sitting her on the toilet as part of the bedtime routine & she often goes but I think this is more due to coincidence that if you were to sit someone on the toilet for long enough then of course they will go.

I put her in knickers a few months ago (before my son was born) & tried the Gina Ford approach with sitting on the toilet every 15-20 mins but she went just after she'd been on the toilet & wasn't in the least bit concerned that she was wet.

I'm starting her again in knickers again tomorrow but don't know how often to put her on the toilet. Every 30 mins? Every hour? I'm really hoping that this time is it!