This really depends on your usage, but if you are not sure it's always good to start low (e.g. 50AED) and you can adjust your 'Auto credit' according to your usage. This will keep you safe in the knowledge that you will never have a zero 'Wallet' balance. Pretty cool, right?
Got more questions about the 'Wallet'? Check out 'What do I use the 'Wallet' for?'