Boost Membership Pending Status: How Long Before Funds Are Released?

Does anyone know how long a Boost member sits as pending on the account before being removed and the funds held for them are freed up? In case that doesn't make sense, say Bob (fictional) signs up for my NL via a web boost. The $1.50 I pay per lead is technically no longer available in my budget/funds. But they haven't confirmed their subscription yet. How long does it take the system to determine they're not successful and free that $1.50 up for a different lead (sorry, Bob!)?