With the proper care, the skin should last about ten years, however this does depend somewhat on the type of skin you have on your instrument.
Most important is the proper storage and protection of your bodhrán when not in use or during travel.
 

1. Keep it where you yourself would want to be kept, in a moderately warm dry place.
2. Keep your bodhrán away from any extreme temperature's like over the fireplace or in the cellar.
3. Don't keep it near a lot of water unless you live on a very small island.
4. Don't keep it locked up in your car or on a window sill.
5. Don't let it get too hot or dry, the skin may tear.
6. Don't travel with it in a plastic bag, get a proper material or leather bag made or a wooden or aluminium case.

Rub a little leather preservative or softener onto the outside of the skin every now and then to keep your bodhrán skin in good condition and lasting longer. You can use dubbin, Neutral (uncolored) shoe grease/fat or neatsfoot oil for this purpose.
I use Dubbin, its relatively inexpensive easy enough to get hold of and good for the job.

Some people reccomend a special solution of equal parts of neatsfoot oil (a leather softener), liquid dishwashing detergent and water. you'll have to mix these together very well to get a smooth emulsion before you rub it in to the skin. I have been unable to prove this is better or worse than any other.  

Never oil the inside of the drum  

Good care and maintenance should give you trouble-free playing for many years - good luck!