Assuming it's not the cable, sounds like the battery. Although, when mine gave out a couple of years ago, I could tell by the slow crank and the drop in battery voltage. It did go pretty fast after that though, I could barely get the car started on my way home the day I bought a new battery.

You may also want to check and see if the charging system is working properly. With the engine off, battery voltage should be around 12.5-13v, with the engine on around 13-14v (from memory).