This is what I received via my email inquiry regarding how long the SIM stays active with Lime:
"the expiration time period begins based upon your expiration date of your last top up. After the expiration date has passed; there will be a 120 day period in which you will be able to top up and still retain the credit. After that you will have another 60 days to top up and if this is not done, you will lose the service entirely."
So the expiration period of the last top up would be 30 days, 30 + 120 + 60 = 210 days before your SIM would need replacement. I did see a post here a while ago where someone said that it was almost a year later (since their last trip) and they topped up online just before leaving and everything worked great.
There is a LIME desk in Mega Mart in Montego Bay. I stopped there the last week in November 2012 to get supplies and discovered the desk there and bought my phone CHEAP. It was $1000j for phone, sim & $100j in minutes. If you have a private driver, tell him you want to buy a phone/sim on the way to Negril. He should quickly be able to find a way to place a phone or sim in your hands.
SHEBA was it a digicel sim that you couldn't find?