From mid April to September, but it is Scotland, so you never know
Start of the walk is in the small village of Arnisdale. The path up is very steep but gives great views. Be careful since the last part of the path is almost a scramble. The very top is a great viewpoint. If you are fit, you can make it to the top in just two hours. I recommend to take second path on the way down. You will get great views to the west. Beinn Sgritheall is 974 meters high and is one of the Munros.