I stored my Raleigh Superbe in the fall (October) and in the spring the tyres had only lost a few psi and were still within their operational range... I attribute this to good quality tubes and some puncture seal.

I will be checking the herd when I get home and see which wheel sets have maintained the best pressure through the winter.

Schwalbe tubes seem to have the best retention of any tube I have used... with these I find that pressure losses are significantly reduced.