Publilius Syrus said in 100 BC that “Everything is worth what its purchaser will pay for it.” This is true for your photography too. When you’re starting out you’ll mostly be giving away your work for free. The first reason is people won’t want to pay you, the second is people have higher expectations for their product when they hand over their hard earned money.
When you should start charging money is when you no longer have time to fit in all the (time for, or trade) requests you’ve been given. Politely let the person know what your rates are and that they are negotiable.
You probably want some hard numbers. Many photographers I know in the DC area (which has a relatively high cost of living) charge about $75 for a location shoot with about 5-10 finished photos. The photographers charge substantially more for studio shoots, going for around $150 for about 5-10 finished photos.
However if you feel overwhelmed with TF requests you could certainly try sending your rates to those who don’t have the looks or quality you’re looking for. What rate to charge is up to you, but if you’re new to the game $25 an hour is probably a good place to start out. You could also try charging for extra services such as giving models all the proofs.
Some other photographers I know have a sitting fee of around $50 and then charge about $25 for each finished image the client wants.