Phew! that sounds heavy going! It's even slower in summer (same hours, more elapsed time) as I tend to do more 'other stuff'. I usually manage 2 a year. There's more of that pesky good weather this week so the cellar doesn't hold the same attraction...I’ve never counted, but I do recall that the local crew were quite impressed that I could build two locos per year, when I first found the joys of kit loco building.
13 weeks suggests you could do four in a year, so I’d not be worrying much about slowing down.