Okey dokey!
For Bronze you must do 45 mins spread over 3 session for 4 weeks.
For Silver its either 6x 15 minute sessions or 3x30 mins every week for 4 weeks
For Gold its 10x15 or 5x30 minute sessions for 8 weeks
Platinum is when you include exercise as a natural part of your lifestyle (presumably to gold standard plus)
Hope this helps! We have Fit logs to fill in, (are you going to class?) and we just say when we've made the grade. I fill mine in, but they don't check them, it's taken on trust. I'm getting my bronze and silver on Tuesday, our leader has really been pushing this over the last couple of months.