I don t agree that you both need t have exactly equal 'time off' at the weekend...no need for calculating it to the nth minute, but the balance does need to shift.
You both need some 'me' time, plus you need time as a family. Sit down and talk about where the right balance lies...so he feels he gets to pursue his hobby,my , you get some time, plus you enjoy your time together.
Would it be possible for him to just do 1 full day of the hobby per month, plus another half day or couple of half days? If you also had a day a month and a half day, that leaves at least 4 days left (1 per weekend) to be together. Personally, I'd want more family time and wouldn't need full days off very frequently, but a couple of hours in bed,mor in a coffee shop are nice..
I had a similar discussion with my DH when kids were small. His hobby required going away for weekends or whole weeks because it couldn't be done locally, so that involved some dispute. It was hard because he loved it and I knew he did and I wanted him to be able to do it and he wasn't wanting to be away all the time doing it, but found the adjustment to being a parent and loss of freedom pretty hard with a new baby, who to be honest, neither of us found very rewarding. It was made more difficult by the fact I didn't have a hobby....so any 'me' time would be spent in bed, at the shops or in coffee shops on my own or with a friend. We came to agree that if those were my choices they were equally worthy as the hobby, even if they weren't something so specific.
In the end, I think we agreed it was reasonable for him to have 3 weekends per year, plus a short week (4 or 5 days) away. He knew I found being at home at the weekend hard and didn't ask to do more even when he had invitations from friends to go on fun weekends. Many of my friends thought he was away too much and they would never 'allow' their DH to go away so much or at all....however, it wasn't about 'allowing' for me. I wanted him to reach a decision about what was reasonable and fair for us all, that HE had opted into,mrather than being told, which causes resentment. I couldn't bear it when I heard other men talk about being given a 'pass for the weekend' as if they had no say over their lesidure time, which also implied that their wives were old bags, who the rest of the time didn't let them out.
It's all about you working and discussing together and finding compromises you can both buy into, for this new stage of life. Babies are hard and everyone needs some time off and away for their own sanity. More than ever tough, you also have to think about each other and not just yourself. It's hard.