My girlfriend and I did this trek fairly easily in 4 days. (Day 1 to Ulleri, Day 2 to Ghorepani, Day 3 to Ghandruk, and Day 4 back to Nayapul)
A guide is not necessary (we did not have one). The views were amazing! We elected to walk up to Poon Hill from Ghorepani in the afternoon on the day that we arrived, instead of the next morning at sunrise (had lots of time, and didn't want to add another 1.5hrs to our longest day already (about 8 hours to Ghandruk).
A couple of words of advice:
Most places request that you order dinner before 5 (you can schedule the meal for a later time)
Save yourselves some time and order breakfast the night before.
Take 5 minutes to climb the stairs to the right once you arrive in Ghorepani, and check out "upper" ghorepani, the views are significantly better, and the town looks quite a bit nicer there. We stayed at the "see you inn" in "lower" ghorepani, and it was the least nice of all of the houses we stayed in.
If you are debating stopping in Tadapani instead of making it to Ghandruk, push through, it is only about 2.5hrs more and all downhill. There are a lot of very nice housing options in Ghandruk (we stayed at the Hotel Trekkers Inn, which was very nice.
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