Most people I know are not paying anymore. But if you decide to pay:
If you have paid 40%, wait until the project gets to 40% progress.
if you have paid 30%, wait until the project gets to 30% progress.
if you have paid 20%, wait until the project gets to 20% progress.
I would not pay more than 50% to anyone in Ajman. Let them give you keys then you pay the rest.
ARRA chief Mr. Barghouthi said it many times, payment is only for progress in development. if your developer gets his money every month anyways, he has no reason to make progress.
you know ajman developers by now very well.