There are a lot of potential/related questions here and it's unclear exactly which one you're trying to ask.
How many hours am I allowed to work, maximum?
Your last question covered that and it's mentioned in another answer - 40 hours, by state law.
When someone says "full time," how many hours does that usually mean?
That depends on who the "someone" is. Culturally, in the US, full time generally means 40 hours (from the practice of working 5 days at 8 hours each day). Legally, in the US, over 30 hours generally means full time (in terms of the employer having to offer you the "full time" benefits package, including healthcare, etc.) Further, a company's HR policy may specify a specific number of hours expected from full time employees, i.e. 37.5 hours (5 days of 7.5 hours each).
Finally, your question may be read as,
If I check the box on the application indicating I want a Full Time status, how many hours will I likely get?
This is a hard question to answer on-topic here, since it is dependent on that specific company's policies. However, there are a few general trends, at least within the big-box-chain retail sector:
- Employers are generally reluctant to have actual full time staff, since that means they need to spend a significant amount of money on benefits (healthcare). For this reason, it's common for these employers to have a large number of part time staff
- Further, the full-time benefits package is often seen as an investment into a specific person, so it's not often given to employees unless the company considers them valuable in the long term. A younger person applying for a summer job probably won't fit this criteria - they'd rather give the benefits to someone experienced looking for longer-term employment.