We drove Ullapool to Thurso, June, 2015. This was a day trip - we were heading to Thurso to take the ferry. If this is your first trip to Scotland (it was our 6th), I would not make that drive a priority unless you want to go to Orkney. The area is very bleak, mostly treeless, and there are almost no amenities - be sure you have a picnic, a full gas tank, and a giant bladder! The road is single-track with a passing point every few miles which is OK since there is almost no traffic.
IMHO your time in Scotland is better spent driving the Great Glen, Glencoe, Skye, Lewis, Mull, and the border abbey areas.