[maxsavin]

I usually charge per day or per week. If it's a few hours, I just tell them it's not worth my time.

The locality of the customer matters a lot. Usually, I will factor what it would cost for them to hire a full-time engineer (not just salary, but also overhead). A $120k salary will cost the company $200k, or $90 per hour. Then I'll add a % on top since I'm not getting days off, paid vacation, long-term security, etc.

That kind of reasoning helps with stingy customers. Otherwise, I just give a flat rate to solve their problem.

For customers that need me long term, I also ask for a monthly retainer to keep availability for them, for meetings, emergency bug fixes, etc. Otherwise, you're giving them the perks of an employee without them properly compensating you.