How much you charge is dependent on the work that needs to be done and the results you tender. Kind of hard to put a price on an hourly basis without more information.
I can mow my own lawns that are about two acres and trim the fences and trees in about three hours. String trimming takes most of that time I should point out, not much fuel cost just time consuming. On a regular basis I would charge about $150 to maintain a similar lawn. On lawns without a regular schedule and likely taller weeds I would charge $250 or better for this.
If you were to mow my lawn with a pushmower it would add several more hours to the job to accomplish the same results. So assuming you could mow and trim it in six hours each week you could still get about $20 to $25/ hour or $100 to $150 total for the job. You need to get enough to replace the equipment when needed.