I'm a first-timer, building a Snow Goose. I guess the answer is ultimately determined based on your skillsets (woodworking, epoxy experience, etc.), but based in my SG experience I'd say it is not too much for a first-timer. I've been doing woorkworking in various forms for many years, I'd label myself as between moderately skilled and advanced for a hobbyist, but had no epoxy nor boatbuilding experience. I grew up on the water, so also had a good handle on boating in general, primarily smaller (<20 ft) craft.
If you look at my SG build thread, you'll see I've been at it much longer than 5 months - I'm on track for about 18 months +/-. I need to try to estimate my hours, but I'd say 5 months is doable if you have a good skillset and can throw the hours at it. I'm retired, but have taken extended breaks for vacations, etc. and don't work on it every day. I have put in a few full 8-12 hour days, but most days 3-4 hours. Epoxy and glass work is slow on your own (mixing in small batches, etc.), but the biggest slowdown is waiting on epoxy to cure. As a first-timer, you'll also have to build in time for experimenting and figuring how to do things.
Devlin's book is a real help, and I'd also recommend the WEST Systems pdf on epoxy if you aren't experienced with epoxy. And definitely start a build thread, you'll get lots of great advice and support here. Looking forward to following your build!