I'm planning to take my first guided fishing trip and was wondering if it's customary to tip the guide and if so how much? It would be a2 person half day trip. Thanks in advance for any advice.

Two people and you like what he's done for you then I would say 30-50 bucks.

If your guide shows up prepared.....rods tied....bait caught...ice for your fish....has a place to clean fish....
Basically all you need is snacks drinks and your license......then he's done a lot before you ever got there.
That's why you hire a guide!
Make him look forward to your next phone call by compensating him well.
It's a win situation for both of you.
15-20% is a good place to start.

There are guides and then there are guides. I had a young lady tell me recently that she had given up on guides and only came because her husband made her. She tipped well and said she was defiantly coming back after fishing with me. My point is go more than once and find a guide you like that can catch fish.
All guides have a bad day every now and then. That's not the norm.
If you find one you like build a relationship just as you would with any service based business.

I usually tip 25 dollars, unless they are illegals. I dont tip illegals

I have tipped anywhere from 0 to $100 depending on how much fun I had. Note, I said "how much fun", not how many fish.

Very well said Randy!!