Hard to estimate, probably a few hundred hours from me, and a comparable amount from others. As a random data point, I sent 53000 messages in the group chat developing it. ¯\_(ツ)_/¯
You're very lucky to have that. I've always wanted a circle of programmers to build stuff with. Its an incredible force multiplier and I'm sure you'll have fond memories of that camaraderie for life.