There's no right answer for that my bro. Lift as heavy as you can while falling in your target rep range and using proper form. If you're training for hypertrophy, you should be coming to about 2 reps shy of failure.

Is it beneficial for me (gym newbie with quite visable stomach fat) to be doing ab work?

I've always been told there's no point until you lose that fat otherwise it just bloats you out. Please help

You should be training your abs bro. You wouldn't stop training your biceps because they would push the fat out, right? Same with abs. It's not going to be bloating. They will get bigger and blockier if you train them right, yes, but not to the extent where your stomach would become massive.

Should you do flat, incline and decline bench press the same day, since all three work your chest? Or can you do them on three separate days?

You can do them on the same day, or seperate days. Whatever works best with your workout scedule. If you only have 1 chest workout day per week, and you want to hit all areas, then probably do all three on that one day. If you have two chest workouts per week, you might just do a lot of flat pressing on the first day, then incline and decline on the second day. Or some other variation.

Whatever works for you! Just make sure you have enough total volume for the week (around 20 sets would be a good target for the week).

If your primary goal is to build muscle, is it best to do your workouts in a circuit, or just do one exercise at a time?

One at a time probably.

You can do supersets if you want to tie some exercises together and do them back-to-back. You can do circuits to build muscle as well, but it's more for endurnace, and while gaining muscle isn't always about lifting the MAXIMUM weight, you still want to be giving each exercise and set a lot of effort, which is easier to do when you are doing one exercise or two exercises at a time.

How long should a workout session last for a beginner?

Probably 45-60 minutes is a good target for a beginner, but that's not to say you can't be in there longer. If you have short rest periods, your workout should only be around 60-90 minutes max anyway even when doing something like the PPL or Cheat & Recover programs.

How many sets should a beginner start out with?

15-20 sets for bigger muscles groups like chest, back and legs. Closer to 10-12 for smaller muscle groups like arms. Then you can increase that volume as you become more advanced! Check out the PPL program for an example!

How often should you train your abs as a beginner?

Hey S133003836,

That depends what routine you're using. A general guideline which works really well is every 48 hours or the classic Monday-Wednesday-Friday approach. Abs can typically be done with higher frequency and it's easy to recover from. I say blast them at least twice a week, and more if necessary.

I agree with Alex. Abs can take a beating man. Abs can definitely handle being training every second day. I like to train abs 4 times a week myself!

What are some other exercises that work your lats besides lat pull downs?

What are some other exercises that work your lats besides lat pull downs?