When is a good time to text him again?

k

I caught up with this guy I used to talk to. I’ll call him a friend because we never actually dated but there was a romantic connection. Anyways, he texted me Tuesday for my birthday and we talked for a bit. Towards the end of our conversation, he said “Drop in and say hi every once in awhile and I’ll do the same!” I want to text him again but….is it too soon? How often is “every once in a while” lol