Travelling by....train? Tube? Car?
The tube journey is at least 60 minutes, from the train actually departing. It's slightly quicker by Heathrow Express plus tube, but not much. Road travel depends on time of day.
If you're trying to book train tickets, my advice would be think of a number and double it, then add an hour for safety
There's some decent cafes etc. at Euston nowadays, so it's not the grimmest of places to have to kill an hour or two.