as a freelancer ,in a normal situation they charge hourly or fix rate per project but sometimes came a situation when the issue is very critical but the solution is very simple , like a small change in web config file or just modify some select query. basically you hardly need 2 to 5 mins to solve things. in such condition, how would you suggest to charge to client? P.S. skip the situation where he/she is your regular client and you have a good relation with him/her so you would do it for free or nominal price.
Ravi Khoda Humanity is the best religion and smile is the best medicine.