I'm not sure how long it takes, but if you check out this Longwood website it lists their excursions and the prices. You can certainly do Cairo in a day it seems - and even go by air if you want rather than a long coach trip.
We stay in Nuweiba and from there Cairo is about 5/6 hours by coach, and the cost is a bit more than these trips from Hurghada, so I'm guessing it's a bit shorter.

The Longwood website is a good guide to the trips (& prices) with other travel companies as well.

yes it is a long trip from hurghada to cairo .. aprrox 5 hours each way on the coach makes for a very long day

we did the trip by air which took approx 45minutes, it meant we had longer in cairo and didnt have to sit on the coach for all those hours... was a great day and well worth doing, we just booked through our tour rep

Bear in mind however that if you are diving you should not fly (even internally) within 24 hours of completing a dive (DAN recommend 48 hours after multiple dives) which may need some careful planning...............

Have undertaken the trip by road from El Gouna (25 mins further North than Hurghada) to Cairo and it is a bearable if rather boring journey (very little to look at) of around 5 hours. Unfortunately there is no rail alternative, the only rail line running from Luxor to Cairo.