The most important factor is what stage the company is at. Since the company is doing very badly and will likely fail you deserve very little credit if it doesn

The most important factor is what stage the company is at. Since the company is doing very badly and will likely fail you deserve very little credit if it doesn't. You do deserve something though. I'd say around 5% would be fair. That assumes you will act in good faith to hand over any IP assignments necessary, etc.