According to this answer, in the event of an explosive decompression it can take as little as 9 seconds to develop problems without an Oxygen supply.
How are cabin crew meant to get to an oxygen supply when they're a long way from their allocated seat ? There's not only distance to consider but if they're jammed between two carts in the middle of meal service they're not going anywhere too quickly !
There are usually extra oxygen masks in the cabin- it is a regulatory requirement. From 14 CFR 25.1447 - Equipment standards for oxygen dispensing units:
(c) If certification for operation above 25,000 feet is requested, there must be oxygen dispensing equipment meeting the following requirements:
(1) ... There must be ... at least two oxygen dispensing units connected to oxygen terminals in each lavatory. The total number of dispensing units and outlets in the cabin must exceed the number of seats by at least 10 percent. The extra units must be as uniformly distributed throughout the cabin as practicable.
So the cabin crew can use the nearest unused oxygen masks. Also, each flight attendant position has a portable oxygen system, which can be used in case of emergency (though it is intended usually for therapeutic purposes). in addition, there is also the Protective Breathing Equipment in the cabin. Though these two are not intended for normal use during decompression, I guess it can be used in case of an emergency.
