It looks like you should give her a prioritized list (verbal or written). Put what is most important at the top, and the least important at the bottom. She will get to all she is able to within her 2 hour time working from most important to least important.

If you want the microwave wiped down inside and out, or parts of the couch vacuumed, she may not have time to clean a bathroom. You're leaving it up to her to decide what she will do with the list of chores you have asked her to do within her time span.
If you are giving her a list that would take more than 2 hours and expect her to do them all in 2 hours, that is inconsiderate on your part. You can't just keep adding more chores to be done within a 2 hour time frame and then be upset that some of the cleaning is shoddy, as she has had to work quickly and not thoroughly.

Nor can you keep adding chores and expect her to stay longer without any extra pay. If you want her to stay for 3 hours, you pay her $75.00 plus her tip. $25/hour is a good price. If you take advantage of her low price, she'll turf you. You may think you've got the upper hand, but you don't. Treat her well.

You will find people are fair to you, when you are fair to them. Appreciation goes both ways.