Shelf Life of Tape Drives?

I have four tape drives, two external, and two internal. What I am trying to do is figure out how long I can go with these drives before deciding to purchase new ones. They’ve been here before I was hired, and I’m not even certain how old they are(I’m sure the external ones are at least a few years old). What signs do I look for that the drive is going to die on me? Lately I’ve been getting a lot of “cleaning” messages on two of the drives. I use the cleaning tapes on them, and then they just ask for it again a few days later. Any care and maintenance tips anyone can give me? I’m not a hardware person, but am always willing to learn. Thanks!