Depends on the care given to the lawnmower. Do you clean the undercarriage after every mow? Do you make sure the engine in not gunked up with dirt? A mower kept in good condition should last at least 10 years.
I had new lawnmower paid 99$ on special sale for 10 years . I changed engine oil every 25 hours, cleaned under the mower with a garden hose every time, oiled the wheels bolts and had it in my garage.
It depends on how often you use it and how you are with it. If you take good care of it and check the oil and change the spark plug often enough I would say it shoulld last you quite a few years. However, if you don't then it might only last a year or so. Also, depends on the type of lawn mower you buy. I would say some are better than others. Good luck!