How long can a broker hold a deposit if a real estate deal falls through and the seller refuses to sign the release?

Unfortunately, the broker legally can't release the deposit without the agreement of the seller--that's a protection to prevent one party or the other unilaterally making off with the deposit, and the broker could get him/her/itself in trouble for releasing the deposit without the signoff of both sides.

If you can't negotiate to something mutually acceptable with the seller, you may need to litigate against them to force them to release the deposit. (You would be bringing a contract action, showing that under the terms of the contract--the contingencies--the deal is off and the deposit should be released.) Since that can be expensive and time consuming, you might want to consider agreeing to allow seller to retain some portion of the deposit, to salve their pride and compensate them for time they invested in the busted sale. That might get them to release the balance.