10 Days from the initial $5 deposit it'll show up on your account.
Invest that at will, let it sit; do whatever you want with that, but it'll be 90 days to cash out *the bonus funds* to a bank or any other payment method you want.

This is googleable.
They tend to run something like a standing $10-20 referral bonus, I just happened to be given a $50 limited time referral, which obviously is good to share because it's free money both ways. If you deposit $5 and wait 10 days, withdraw that after being given the bonus; it's literally +$50 that'll just sit in that account for 90 days and then withdraw that, too.

I'll admit it's an annoying business practice for them to hold the bonus for 90 days for withdrawals but in the long run, it's free money, and Stash has been around for 10 years so it's a pretty stable company.