How do you decide when to harvest?
If you do not want to chop then don;t, full respect, your game. The AI does not really chop and chopping speeds you up so for faster games thats when you use it. Pure and simple.
Chopping in population is also key, delaying your chop of pop can get you to a 10 size city (for rationalism)... but chopping to get another district in by getting to 7 is also a consideration. Chopping with a low pop is a bit of a waste.
here is some general rules I use that should help you manage the decision should you choose to embark.
Rule 1. Chopping is a limited resource so if you have few you have to chop less and if you have more you can chop more. Just be aware of builder costs. Good chopping ground mean wonder territory.
Rule 2. harvesting early gives much less reward however it will push your civ forward so the key is to harvest what needs to be done early for your gameplan. For example pyramids is built by the AI always and fairly early so if you want to get it you better be quick and chop. It also helps get to theaters faster.
Rule 3. Chop at the end not the beginning of a wonder. That way you do not lose the chop and the wonder.
Rule 4. Later chops are stronger so if you have one chop to do in a city and have to build both a library and a uni then chopping the uni later is a better option... but not always... if you have 3 science CS and 3 envoys in each that is a lotta science to your library.
Rule 5. Avoid too many chops until your builders have more charges, builders increase in cost and they can get mighty expensive at the end if you go crazy.... so ideally chop after feudalism but maybe not immediately as there will be a few cheap civics that can quickly up the chop value... you judge.
Rule 6. Chop all your crabs and copper near the end of the game, the money you get is crazy and you will get little off it for the last few turns.
Rule 7. Chopping when you have the harvest goddess is awesome in combination with the golden age monumentality, you really have to try it some time to be disgusted by the results. Just one chop can provide enough faith to buy another builder... and you can buy settlers with the faith too. It's just crazy.
Rule 8. R&F overflow chopping is gone so do not bother learning it. But chopping with a +50% or +100% production will increase the chop for that type of unit/building/district. The +15% wonder card when chopping wonders does help, as does having autocracy.
WARNING: If you are OCD then chopping may completely change your view and you want to clean up the entire countryside of all that dirty stuff. Try a little chopping, put Magnis in a city for 5 turns then chop in a wonder. It is wonderful, you hover over the chop button to see how much you will chop for and also you can see how mch a wonder is so you can assess how many chops and when.