I would leave it 48Hrs ...
That seems very, very short.
Watching my brickies recently take off a couple of courses where
they'd made a mistake on a chimney stack, 2 days after they'd gone on
- they had not the slightest difficulty in such green mortar - still
very easy to clean off.
ISTR - a long, long time ago when I worked for Costain - the cure time
for mass concrete (in the Summer) was 4 weeks after the shutters came
off. Certainly when I'm using concrete, I'm surprised how easy it is
to chip for the first week or two - compared to how hard it gets
eventually.
In the current cold weather I'd plan on doing some other job - before
putting, as you say, a significant sheer load on your new courses.
You might find useful manufacturers data here - certainly their advice
on concrete I found useful:
http://www.lafargecement.co.uk/site_...ge/default.asp