yo, im 18 as well and have a ninja. I cruise around 7k to 9k rpm when going 60.
i got around 63 mpg for the month of october.
30k - 60k sounds about right on the 250. After that many miles, you'll probably want to step up to a bigger bike anyways. But, an engine rebuild would be in the works at that mileage. Cylinder overbore, new pistons and rings, timing chain, timing chain guides, seals, bearings, valve job, and new clutch plates are the least you could expect.
Like a car, you'll need regular oil changes every 3,000 miles, new tires periodically (bike tires only last about 5,000 - 8,000 miles or so,) new drive chain and sprockets every so often (maybe 10,000 to 15,000 miles, depending on how often you manually oil it.) Spark plugs and brakes every 6 - 10,000 miles, you have to have your valves adjusted in the same interval. Every couple of years, have your dealer or shop go through everything, check the battery, lube the cables, adjust chain, check steering head and wheel bearings, etc etc.
The just keep the bike running healthy and you never know how long they can go.