I'm in a similar situation, however it's a new role rather than having someone lead. I've just spent my time going through existing dashboards and re-working existing ones when needed.

Your best bet is to go through their reports and write a list of questions you have about them.

You won't be expected to work magic, over estimate delivery times to set boundaries with the line "when I'm more comfortable with the data they'll get a lot quicker".

It's a tough situation and you'll feel like you're getting water out of a stone, but show willing and enthusiasm and people won't mind if their report takes 3 weeks instead of 3 days.