Under UAE labour law a "working week" is defined as 48 hours, how this is achieved depends on your contract and sponsor. Some companies have a two day weekend, others only one day off.
If you work in the retail, security or entertainment industry in the UAE the working week is 54 hours, after which OT is paid.
OT, by law is calcualted at normal rate + minimum of 25% (yes only 25%) if the OT falls between 4am and 9pm. If the OT falls between 9pm and 4am the rate is calculated at normal rate + a minimum of 50%.
This is the legal guideline, but implementaion may vary from company to company - check that this rate is covered in your letter of appointment!
I'd check on the vacation time, the norm here is 30 days per annum. You need to check when you can take it, some industries in the UAE give 60 days and a plane ticket every 2 years, others give 30 days and a ticket every year. Under UAE law your employer is not obliged to pay for your leave ticket each year, although most do as part of the "package".
Also note for your first 6 months of employment you do not accrue any leave benefits! this means that if you leave before your first 6 months is up, don't expect to be paid out for it! Leave can only accrue for a maximum of 2 years. If you don't use it you loose it!
if you are scared by the idea of a 48 hour week then don't come to Dubai, most people come here to work and make money! if you don't work hard at it there is always someone else ready to take your place. although the adage "work cleaver, not hard" is true here!
personally i put in about 72 hours a week - but then i choose to