Purely my thinking: everyone is different but if you're say a genuine mid-pack fellrunner with some ultras / MMs / long fell races under your belt then you have a chance of getting round on one decent run (say 5,000ft-7,000ft, 4-6hrs) per week for maybe 8weeks (plus other training). I'm around the last 20-30% in races and had to throw myself into the BG to have a chance (12weekends running on Sat / Sun, 150,000ft Jan-end April) . Very strong runners could get away with a run as above per fortnight but you'd have to be exceptionally talented / strong to do one long run per month!

Not being injury-prone or susceptible to blisters, being able to eat well on the hoof, becoming obsessed by rhe challenge, simply loving the mountains, having a strong mountaineering / hillwalking background and / or being mentally tough can be all positive factors in whether you succeed too. I suspect you'd need at least one of these in your favour however strong a contender you were to get round