Though I suppose it's too late now, answering the question won't hurt. To answer your question directly before going into detail, the average gestation period for female RES is two months, and no its preferable you don't leave your turtle in that state, though she may be at the beginning of her gestation period. A way you could know when your female is ready to lay her eggs is when you notice she is spending more time on land, sniffing and digging constantly to find a place to lay the eggs. This happens during the last two weeks. At this stage you need to transfer her very carefully to the nesting site which you should have prepared by now.
Even if she is at the beginning of the gestation period, she is still at a time where her eating habits would change and extra care is required for her. At this stage she will probably start spending more time basking and extra calcium and UVB rays should be provided for her. Also, she might start wanting a specific kind of food and will start rejecting a lot of other food. Close attention should be payed to her diet and to what she is preferring so it is constantly available for her.
The first thing you need to do is make sure your nesting site is appropriate, which currently it isn't. Just keep in mind that the eggs maybe infertile as the female may be gravid even if the intercourse was unsuccessful. Though some websites say you can make a nesting area of about 4 inches, it's preferable you make it much deeper, of about 12 inches, though I think this number is a bit exaggerated, it's just for being on the safe side. The nesting site should be made of sand and organic substratum. It is imperative you remove all obstacles from the way as the female may stop digging if she feels there is something blocking the way.
Hatching occurs between 60-80 days from laying the eggs but may be several days earlier with controlled conditions. Watch the turtle in order to know where she left the eggs. If the nesting area is appropriately set in a good basking area then their is no need to remove the eggs. Still, Incubating them makes it easy to monitor them and avoids any fungus spreading from one egg through the nesting site. The container for incubation has to have holes for aeration and the temperature should be maintained at 81-87 degrees Fahrenheit. Removing the eggs from the nesting site should be done very carefully and you should mark how the eggs where placed in order to place them identically in the incubation box.
Remember that if these eggs are fertile then you are adopting a very big responsibility and a ton of research and preparation for the juveniles should be done. A main precaution I'd point is to leave the hatchlings in a separate container with shallow water and an easily accessible area of land for basking. I hope this info helped, but I still advise you do a lot more research as the wrong actions may damage your RES very heavily, a common consequence would be egg binding. The eggs will eventually rot or calcify in the female, and can cause serious diseases and bacterial infections.
Finally, I'd say you don't leave her in this state. If you have someone relatively informed of turtles leave her there and inform them of what to do. Or you can contact a vet and let them care for her during those days. If you leave her like that she might lay eggs and retain them, and eventually her eggs might bind. You should prioritize this turtle's needs as her health now depends on it, especially if she's near her laying stage.