If you begin your Poon Hill Trek from Kathmandu and want to end in Kathmandu, it takes at least 6 days and if you want to spend a day to explore Pokhara, it total takes 7 days for poon hill trek. Actually, a weeklong vacation will be enough for Poon Hill Trek in Nepal.
If you begin from Kathmandu, the first day, you will transfer yourself to Pokhara by air or by road. If you use road transportation, it takes more than 7 hours to get Pokhara from Kathmandu. Or, if you use air or domestic flight, it takes only 35 minutes.
The next day, you will drive to Biretahnti and trek to Ulleri and on the third day, you will hike up to Ghorepani village ( 2880m). On the 4th day, you will climb Poon Hill and trek to Tadapani. The fifth day will be for the walk to Ghandruk Village and the sixth for retunring to Pokhara and the final or 7th day will be for returning Kathmandu. This is the way you spend all 7 days or a week long days for your Poonhill Trek.
However, some people try to do Poon Hill Trek even in 3-4 days which will be very rush, but still can do. You can use the domestic flight to go to Pokhara from Kathmandu and then begin the trek on the same day. The next day, you will get to Ulleri and then to Poonhill and you can come back along the same trail back to Pokhara on the third or 4th day. This way also is possible if you relay want to do Poon Hill Trek. And fly back to Kathmandu on the same day and catch your international flight at night to homewards.