How many miles can i expect to get out of a 98 chevy cavalier before i have any major repairs to worry about?

Depends which engine is in your car. The cavalier had 5 diff engine models. You will need to check part numbers. Most bolt on and maintenance parts are most likely the same. Major parts most likely much different. Contact your local auto part store and reference the two parts in mind of changing.
Here's an engine list.
2.2 L LN2 OHV I4
2.2 L L61 DOHC I4
2.2 L L42 DOHC I4
2.3 L LD2 DOHC I4
2.4 L LD9 DOHC I4

your local parts recycler (junk yard) can tell you what fits..

I, feel sorry for you, after reading your problem, but don't worry you visit the Reliable-Store they will help you for sure.They have repair manuals at a very less cost. Best Manuals

i have a 98 cav with the same problem, it's not really worth fixing if you know how much it is off by,

That is really quick, like you are getting no water flow at all. Make sure that a radiator hose is not collapsing enough to stop the flow. If possible on that model, take off the radiator cap and shine a light to see if the water passages in the radiator are clogged up with rust or sediment. If this started after the new thermostat was installed, make sure it was not put in backwards.

This model always this problem, usually after about 90,000 miles and the other instruments will eventually follow. There are companies that will repair this for you just use google (instrumentclusters.com) to find them. I was charged 185.00 for the repair... GM wanted over 500.00!

The spark plug gap is .050". Hope this helped and best wishes.

If you keep the oil changed and keep the engine tuned up, no problems. Have the tranny fluid and filter changed also. I've got 245,000 + on my '98 w/ this philosophy.

The transmission needs major service -- it should be transported to a garage for repair.

Well the biggest problems for the 98 and 99 cavaliers are that they mess up from the timing chain,..they make a rattling noise on the right side underneath the fender.