The trail from Jhomsom to Thorong La (La means Pass in Nepali) is a very nice short but very high altitude trek. Actually it is a piece of the Annapurna Circuit. Although this trek is one that is attempted fairly frequently, it is extremely likely that one will be intimately introduced to altitude sickness during the trek. As the result of poor planning most people don't make it all the way up to the top of Thorong Pass (17,650 feet). Most people attempt this trek in about 5 or 6 days round trip from Jhomsom to the pass and back to Jhomsom. Seven days is recommended for most people. The change in altitude is extreme as the trek starts with a flight from Pokhara to Jhomsom flight going from 2,686 feet to 8,900 feet in 20 minutes. Then by foot from Jhomsom (8,900 feet) to Kagbeni (9,200 feet) to Jharkot (11,850 feet) to Ranipauwa (12,225 feet) to Muktinath (12,475 feet), to a single house/tea house (13,400 feet - open to visitors with a couple of rooms except during heavy snow season) to Thorong La (17,650 feet). Planning a week would be a good amount of time to properly acclimate as you go up. The day the pass is achieved from Muktinath (12,475 - very difficult start) or the tea house (13,400 - alot better start) is a hard day but that same day it is possible to go down to Jharkot to sleep and the next day all the way to the Jhomsom airport. A conservative itinerary could be:
Most poeple get to Pokhara by tourist bus (about 7 hours) from Kathmandu's ring road bus terminal. The flight from Pokhara to Jhomsom costs around US$100 round trip and should be booked well in advance.