In reply to JackHatton:
In the right weather, it's a decent days walk. If you haven't done any hill-walking before, I'd suggest the easiest route is the good path all the way from either Achintee, or the Youth Hostel, both in Glen Nevis. There are more scenic routes via the Carn Mor Dearg arete, which would take a bit longer.
Best time of year would be late spring and early summer. Depending on the winter we have, there may still be snow lingering at the top in late spring. Navigation at the top can be an issue in bad weather, so make sure you have a map and compass and know how to use them.
The weather can be a complete lottery and can change quickly, at any time of the year. On a good day, it'll be a good day out. On a bad day, it could be horrible and potentially dangerous for the inexperienced. It's a shame you're so far away, since I presume you're going to have to fix a date sometime before you travel, which may or may not be the best time for decent weather.
Basic hill-walking gear is needed - decent footwear and waterproofs.
When you do decide on a date, post again here - you might get someone more experienced to go along with you.
You can get a bit more info here:
http://www.walkhighlands.co.uk/fortwilliam/bennevis.shtml
http://www.mountainwalk.co.uk/benneviswalk.html