It very much depends on the level of walking you're after. If you are an experienced hill walker there are far more options and you will want to spend much longer on Skye, than if you are aiming for easy low level walks
I'm assuming you are experienced if you're thinking of doing Ben Nevis at this time of year in which case the Cuillins for example offer some amazing opportunities, but you will definitely need very good navigation skills and full winter gear such as crampons and ice axes (and don't go alone obviously). The same will go for Ben Nevis by the way and make sure you check avalanche forecasts as depending on when exactly you are planning to go this can be a serious issue.
It's hard to say how much time you'll need as the weather is so unpredictable and you may have to wait a day or two to do the hikes you really want, but if you take a full week you definitely won't be bored (3-4 days for the Cuillins, 2 days for the Trotternish peninsula and a day to go to Neist point and walk around the coast there (some of this will require a car)).