When we first starting training I would remind her constantly...to the point where she'd get really frustrated with me and be all "Nooooo mummy I don't NEED to wee!!!"
But I was sick to death of cleaning up puddles so I was all over her about it. I reckon every 20 min to half hour.

Now she's good and she tells me when she needs to go but if she hasn't been for a few hours or she's been really distracted playing I still remind her.