I did it in 2 days from Neptune’s Staircase to Dochgarroch Locks in a sea kayak. It was February, and it was a pretty wild weekend. The wind was a howling South Westerly. The rivers were up. Loch Ness was monstrous. It was scary out in the swell.
We B&B’d it overnight in Fort Augustus. I started day 2 by falling into the water trying to put in from the loading dock at Fort Augustus.
At the end I was so knackered I could hardly lift a cup of tea to my lips for the following week.
In summer, with the right weather, plenty of time, and no midges, it would be great. I wouldn’t try another 2 dayer in February.
EDIT: I forgot to add, at one point I lost sight of my partner on Loch Lochy due to being in a blizzard. On Loch Ness, only the odd fleeting sighting of each other as the waves were large. Essentially, you were solo.