2strokes?

It depends on the bike. Motocross bikes like CR, KX, RM, YZ bikes will require a new top end more often than offroad bikes like the KDX, EXC, XC, XC-W, MXC, EC, etc lines of bikes. If you don't ride aggressively, you should have no problem getting more than a season out of a top end, depending on how often you ride. Also, smaller displacement bikes (85, 125) tend to need rebuilding more often that larger displacement bikes (250, 300, 500). I know my GasGas will get atleast two seasons, probably more out of a piston and rings. A guy I know went over 4 years riding 42 weekends a year (and he does race) on a top end in his GasGas EC300. When he tore it down it turned out that the piston, rings and cylinder were all still in spec. This engine had over 1000 hours on it.

What bike do you have specifically in mind?

I'm not sure on the price of a top end - I'm Canadian and I've never owned a Japanese MX bike. I think it's around $100-150 for a piston and rings and associated parts. If you're not riding wide open all the time I would think you should be able to get a season or more out of a top end. Remember, you can also stretch it by replacing the rings earlier, then freshening up the piston at a later date. Hopefully someone that has experience with 85s while jump in here and give a more informed answer.

No it is not true that you will have to replace the top end after every single season. Unless you are racing at the top levels and require your bike to be brand new for each season you dont need to do this, and if you are not a very aggressive rider you wont be very hard on the top end which will save you more wear as well. Changing the top end every season are for those who are riding and racing hard on a consistent basis not for the average Joe weekend warrior. I am sure there are guys out there who will disagree but the fact is that for a recreational rider just looking to go out and have fun this is certainly not necessary. Just like Speedymaniac said it is very conceivable that you can get several seasons out of the top end without having to replace it.

If you're racing every weekend, you could be doing 2 or even more top ends per year at $75-150 each (the low price is if you get deals on an auction site or something).

If you're not racing and riding a couple times a week, I'd recommend at least doing one over the winter each year. If you're mechanically inclined, you can change one in about 3-4 hours. Once you get good at it, I've heard of people doing it in an hour and a half.

It might not always be NECESSARY, but good maintenance will keep the bike running strong and keep you from blowing the motor and spending $300-500 on parts. . .