Once I go online and upgrade my plan; how long is it before the upgrade features may be accessed by my customers

What do you mean by ""upgrade features may be accessed by my customers"? When you upgrade, the plan benefits take effect right away. Which is basically the increase on the account limits:

Monthly Submissions Limit.

Monthly Payments.

Available Space(for you to store uploaded files)

Sub-user Accounts(this feature is for you to share your forms with another account, it does not mean "inheriting" account benefits).

Monthly Form Views(https://www.jotform.com/help/355-Form-View-Limits-What-is-it-all-about).

Total Submission Storage.(unlimited for any paid plan)

Let us know if you have more questions.