How long it will take to charge golf cart batteries depends on what kind of battery the golf cart has, its capacity and the charger being used. The golf cart batteries I used to power my workshop back in the nineteen eighties were 210AH and my big charger was a 20 amp golf cart charger. It would fully charge the batteries overnight. Today I've heard of some golf cart batteries being as high as 230AH and there may even be some with higher capacity. These are flooded lead-acid batteries and AGM which are also lead-acid batteries. These kind will also fully charge overnight.
Some people are switching to Lithium Iron Phosphate batteries in golf carts because of all of their advantages.
Fast charging is one of the great advantages you get with Lithium Iron Phosphate batteries. Most of the 100AH Lithium Iron Phosphate batteries can be charged at 100 amps and some even higher. It's always best to charge slower though. Even charging at 80 amps you could charge a golf cart in a few hours instead of the slow several hours it takes to charge lead-acid batteries, they last thousands of cycles and can be discharged to their full rated capacity time after time. These batteries are also much lighter than lead-acid golf cart batteries, require no maintenance and you only need three batteries for a 36 volt golf cart instead of six lead-acid batteries.
If you're reading this knowing nothing about batteries you're probably thinking why would I do that and give up over half the capacity a golf cart battery has?
You would actually be gaining capacity and better performance. Without getting technical lead-acid batteries only provide their amp hour rating if they're fully discharged at their stated hour rate which you can't even safely do with a lead-acid battery. The hour rate on the golf cart batteries I used back in the day was ten. That means you got 210 amp hours if you discharged the battery at no more than ten amps. Golf carts draw a lot more than that. A motor I tested from a golf cart drew around 55 amps under a heavy load. Golf cart batteries will easily provide that and even higher current, but, the heavier the load on a lead-acid battery the less capacity you get and the more the voltage will sag. The capacity is still there but you can only draw so much at a high current before the voltage sags so low the battery won't power the golf cart. So you might only get 60 amp hours from your 210 amp hour battery. That's only a rough figure but it's in the ballpark. Then there's the issue you can't use more than 50 percent of a lead-acid battery or you damage the battery. Unfortunately most people using golf carts use much more than that without realizing it.
With the Golf Cart Lithium Iron Phosphate battery you can use the full 100 amp hours you paid for and will get over 90 percent of that capacity above 12 volts. What you could do is still use six batteries putting the other three in parallel with the first three and have 200 usable amp hours with a total battery weight of under 70 pounds. One golf cart battery weighs almost that much. Carrying less weight means less load on the motor and you will go farther. Because these batteries can not only provide high current they also hold a very stable voltage you will notice the golf cart gets up to speed a little faster and will go faster. The reason for more speed is with lead-acid batteries the voltage drops quite a bit when you accelerate. The voltage of Lithium Iron Phosphate batteries on the other hand will drop very little, and you will be carrying roughly 325 pounds less weight if you go with six batteries even less weight if you go with three.