The two biggest problems with really cheap bikes concerns the bearings-
1) the bearings are really, horribly cheap and soft, and don't last long even under the best circumstances
2) aside from #1 above, the bearings often aren't really lubed properly, which makes them start to grind even sooner
Spin the pedals by hand now and then and listen & feel for any 'grinding' sensation; that will mean the BB bearings are gone and need replacing.
Do the same for the wheels, especially the rear wheel. Sometimes with cheap bikes the bearings failing will also ruin the hubs, necessitating buying a whole new wheel for $40 or so, but it's not a disaster.