I'd say it's journey time rather than length, so speed will matter. I would say not less than half an hour, but not more than an hour, each way. Too short and it feels like you've not done very much, too long and kids get bored and misbehave (and families are a key market).
The Ffestiniog are finding that the full Welsh Highland journey is too long - hence the shorter journeys from each end being the majority of the offering.
We still haven't travled the WHR from Caernarfon to Porthmadog, despite having lived nearby for 16 years. 7 hours is too long. I'm worried Debbie would be bored and uncomfortable, especially on the return. For that matter, I might be too.
I've done the full length and it is a bit long to do both ways. Perhaps better done one way and then return by bus on the T2 which is considerably quicker (about 45 minutes).
I'd recommend doing Caernarfon-Porthmadog over the other way, as the "semifast" run into Porthmadog (alongside the WHR(P) ) just seems to feel quite fitting at the end of the trip.
I actually think they'd do well to offer as one of their "railtours" a train one way, coach the other option with a hired coach, as I think they may find that easier to sell than both ways on the very slow train. Could save them costs as they could reduce it to one train pair a day (so if you wanted to do both ways it would only run from one end). Perhaps to add to the experience it could be a historic coach.