The work available for a model varies greatly from personal portfolios to corporate.
The answer to how much you should charge is the same as any commercial transaction. Whenever a product is sold, the success of the sale of that product comes down to the demand and perceived value.
I've met models who see themselves as worth such a high rate that they rarely get any paid work. The I've seen models who are willing to work for a reasonable rate and they experience a regular flow of work.
Eg: Model 'A' charges $100 an hour and gets $10,000 work in a year. Model 'B' charges $35 an hour and makes $35,000 work in a year.
Ultimately you've got to work out a starting point, especially if you're new. Some models like to charge more, but what value is the person hiring the model getting? Are they getting any extra value in paying more or are they better of just looking for another model.
Ive had a few and have a few photoshoots upcoming that are paid, i have only found photographers are willing to pay if your willing to do erotic or nude or implied. But average has been £300 plus travel expenses for 3 hour bookings ranging from clothed, erotic, commercial, and nude.