How long Sam can last, and if it's long enough for Sophia to get off is (I think) a more pertinent question
Sure, and that's already been setup with the unsatisfying climaxes for Sophia in her scenes with Liam. Until Sophia takes the plunge with Sam, the story should build up the sexual tension between her and Sam. Sam being of greater girth than Liam is an easy way to increase Sophia's desire/lust. It's almost a given that Sam will satisfy Sophia in ways that Liam is not capable of.
Couple that with Sophia's discovery of Sam's touch and kissing skills in the hot tub, and they combine to push how tempting Sam is to Sophia.
The coup de grace, way down the road sadly, would be paying off Sophia's refusal to swallow her husband's cum by having her wantonly swallow Sam's deliveries. The cherry on top would be for her not only to be shocked to be doing so, but truly enjoy doing so.