Given the context of the discussion, then if not the whole year, then most of the year rather than a few weeks peak summer service.
Capacity for luggage and outdoor sports gear seems to be high on the thinking for the WHL at this time (hence the 153 conversions). While more passengers go to Oban, the service frequency is higher than currently to Ft Wm and the demand both existing and potential for passengers with bulky outdoor / mountain sports equipment for various seasons / activities is greater on the Fort William section North of Crianlarich.
The winter and 'summer' seasons overlap quite considerably through spring in terms of outdoor activities, so if shifting more of these types of trips to rail or growing the market for outdoor activities in the West Highlands is the objective then service enhancements would need to be at least most of the year if not fully year round.