When we started, I asked Indigo how much time he felt comfortable "giving" to Mr. A. He said 2 nights. I then asked Mr. A if he felt two nights a week was enough to meet his needs. He has a very demanding job, and agreed that this was just fine.
As long as hubby is advocating for the children (or you are sane enough in the throes of NRE) this can work for you. If the men decide that the time they are comfy with and is available to them is enough, then go with it!
Since our initial agreement, Indigo has become secure that Mr. A is not trying to steal me away, and has agreed to 3 nights a week. This arrangment is far more flexible than the original one, though. Increasingly, everyone is comfortable with sleepovers, and recently we've started having a group night.
This, two months in, from a partner who felt physically ill when I first started dating.
I have made it very clear to Mr. A all along that anything I present to him is open for discussion and negotiation because his needs are also important to me.
That said, one lesson I had to learn early on is that "fair" doesn't mean "equal". Everyone is treated fairly, but this does not mean equally. Your husband, contributes far more to your life (for now, maybe forever depending on your poly flavour) than your boyfriend does. So yes, he needs more from you. When boyfriend understands this (which mine did, before I even got it!), it makes things go much more smoothly.
So yeah, it's a little awkward having the "which day(s) do you want/are available for this week" conversation at first, but it gets easier and as things settle, you might notice that you naturally slip into certain days becoming the "regular" days.
Just my experience; hope it can help!