How long to tap keg when using primary only

I've brewed many extract batches before but have recently switched over to 3 week primaries and no secondary. At around 3 weeks I rack to the keg and then stick in my kegerator. How long should I wait after kegging to pour the first beer?
I brewed an all Belma hop pale ale 4 weeks ago (it's been kegged in the fridge for one week) and took a sample today. Unfortunately I was not impressed at all with the beer, very unappealing aftertaste. Is 4 weeks brew to glass maybe too short of a turn around time?
By the way the brew clocks in at 5.4% ABV.
Thanks!

I normally am drinking IPAs and APAs in about 3 weeks from brewday, depending on the beer.

I leave the beer in primary about 10-14 days, and then dryhop right in the primary for 5 days and then keg it. It goes right into my kegerator and it's carbed up pretty well in a few days.

A well made beer with non-complex flavors ( not roasty flavors, oak, etc) should be pretty good coming out of primary.

I had my Cream ale in the keg within 4 weeks and I let it carb for a week and it is delicious. It depends if you do high keg pressure and drop, or if you do consistent pressure like I do. Maybe your beer was slightly flat and you perceived this as bad? In my experience my beer always tastes better the more time I give it to chill and carb (up to a point a month would be kind of excessive IMO) Also did you pour off the first pint or so of your beer?

Biohaz,
I actually only had it carbing up for 5 days at 10 psi so it was definitely still a little under-carbonated. I plan on giving it another taste tonight where it would be 7 full days of carbonation. I did pour off the first pint or so but thank you for the suggestion.