Year round, there is no specific time. You just have to see what she (Suilven) looks like in the morning when you get up: Great and climbable? :) She always seems to have a cloud hat on, even in the summer.
Suilven is only 731 metres high, but its one of the finest mountains in Scotland. Its stunning views make it even more fascinating. There is also plenty of wildlife to see, when i was up last year at the end of July and the start of August we saw different species of Dragon flies, colorful hairy Caterpillars, a slow worm and plenty of Beetles ;)
Regarding the visiting time: I was talking to a man one day I met on a hill while out climbing and he said he had been to Assynt twice to stay so that he could climb Suilven and it never happened because of the weather in the summer. :)