Need for fresh media is determined by your growing condition. Just listen to the plantlet does since there are a number of factors that control the outcome. If your vessel has a filtered opening, it may dry out if humidity is an issue. Which would necessitate a replate or watering etc. Or a plantlet might die off for some reason, in which case you might need to remove and/or replate depending on if it releases ethylene etc. Or plantlet density. Maturity at which you want to deflask. I would be surprised if anyone can predict, even if you provide a photo. That said, I and probably others would like to share your progress and see some cute baby phals