As soon as you can reasonably segregate the deferrals from employer assets. Its going to depend on what the actual process for is, but for something on an ongoing basis, their excuse is not going to cut it. Reasonable is as soon as it should be possible given the facts and circumstances.
For example, Owner is traveling and Assistant gets quarantined with Covid and cannot go into the office to mail a check to the plan. The delay is reasonable. Owner is traveling and only deposits once a month because he cannot be bothered to figure out a better solution (like travel with physical checks if they need his signature). Not reasonable.
My rule of thumb, absent an unforeseeable intervening event, more than 3-5 days is not reasonable. If that happens every payroll, you should have figured out a way to improve your procedures.