You get a new bike when:

the frame is cracked or its too expensive to repair (kind of like a car)

when you notice that your body is not enjoying your bike (some people get full suspension if they notice joint pain with their hard tail)

or just cuz!

For me, i had the fuel from 2001-2005, and then i noticed the riding was just different on it. It was explained to me that aluminum frames develop some wear, and they will ride different after a while. eventually, the frame cracked. so i bought a hard tail. then my husband surprised me with a new built up Fuel (trek replaced the frame).

I agree that riding the hard tail will make you ride better. I notice with my hard tail, i move a lot more, and then when i use the Fuel, i tend to get a little lazy. So i actually prefer the hard tail for that reason.

If you can, rent a few decent mtb of various geometries and suspensions, and see if there is some feature you need or are looking for.

Budget will be a big factor of course.

and HAVE FUN!

buying bikes, IS fun. especially buying clothes to MATCH the bike
hahahah