I advocate a twenty minute rule. It's not hard and fast, but shouldn't be more than an hour, and shouldn't be less than 20 minutes.

This is the amount of time the person needing help should try to solve it themselves. After that they should write down what they've done (they should be doing this anyway in an engineering notebook or worklog file), and then ask for help.

The delay is to make sure an attempt is made to solve it themselves. It's long enough to get them to try to understand the problem, and short enough to not put them far behind.

I've also found that if they need to get help often then it's not their problem, but the problem of who is tasking them. Their tasking, or the context of that tasking, is probably not being communicated well, or might not be defined well-enough.