What is a reasonable amount of time to give a customer to provide missing information?

We usually give the customer 1 to 2 weeks to provide us with the missing information.

The key is to be consistent to avoid any Fair Lending issues for disparate treatment.

For the notice of incompleteness, the most common I've seen is 15 calendar days.

I agree with consistency for similar items, but it can vary depending on what you're asking for. The standard is usually about 10-15 days, but if you're asking for their 2014 tax return and it's January, you may need to give more time for your request to be "reasonable". That's why Reg B doesn't define this term.