What amount of Bitcoin would you consider "safe" to keep on an exchange?

I wanted to buy some Bitcoin trough an exchange and then send it to a hot wallet, however the transaction fees are too high for me. I wouldn't mind a 7$ fee if I bought 1000$ dollars worth of bitcoin, but I operate on much lower amounts. So I was thinking if keeping those small amounts on an exchange is okay for now? At least until the fees get lower or I collect more Bitcoin to send a larger amount in the future. I plan to DCA, 25$-50$ dollars monthly for now, maybe more in the future. What do you guys think?