When I got back outside after seven straight months on the trainer it would take me about two weeks to start seeing substantial power increases but my power would be at least equal to what I could do on the trainer on day one. And from the first day outside the RPE was generally lower for a similar effort and time flew compared to time indoors on the trainer.
So in terms of 'getting road legs' it sort of depends on what you're comparing to. Outdoor riding seems easier for the same power and more enjoyable from the first day but I'd typically see fairly big power increases after a couple weeks outdoors so maybe that's when my 'road legs' kicked in.
FWIW in '08 we had a really long hard winter and I raced a 3 day stage race in April straight off the trainer without a single day of outdoor riding since the previous October. I raced well, had a good time trial finished each stage with the lead group and managed 7th or 8th IIRC in GC straight off the trainer. So dealing with things like pack dynamics, surges, sprints, terrain variations, etc. wasn't a big deal even after a huge unbroken block of indoor training. YMMV.
-Dave