I start a new culture every 2-3 weeks regardless because every now and gain a culture will crash so having a constant supply is vital. I have a large salad bowl just for the frogs and put in 1 teaspoon of Nutrobal Reptile Vitamin Supplement Powder, if you hit the fly culture at the base all the flys will drop down to the bottom now remove the lid while holding the jar in your left hand and spin the base of the jar with right hand, this makes all the fly come out into the slad bowl nice and smooth. The powered stops them running away as clogs them up. I give the bowl a shake to coat them in powder which gut loads full of Vitamins then pour them through a 2 piece tea strainer. you should have left a salad bowl with powder in and a tea strainer full of flys coated in powder. At this stage i will feed to my frogs or drop them into a new jar to start a new culture.
If using my resapie make sure you make the culture 24 hours before adding flys, this will allow the yest to react and any gas to leech out before sealing the flys in