most modules run from february - october with a few running from october - june. the first module that you would have to do has 2 starts one in october and one in february, i recommend starting that one next october and then sign up for the follow on in february. modules are done at level 1, 2 and 3 which is basically equivalent to first, second and third year of a regular college. you can see the complete list of modules and compulsory/elective modules here.
you have to gain 360 credits for the degree, 300 of these must be from the courses listed including the compulsory ones and 60 remaining credits can be either more maths or anything you want. if you already have a degree you can use it for free credit transfer to make up the last 60 credits (and save a bit of money)
modules are either 30 or 60 credits - most in maths are 30. OU recommends 8 hours per week on the 30 credit modules and 16 on the 60 credit modules. it really depends on your own ability and experience. The first maths module is about equivalent to leaving cert higher level maths so although i started the course in october, they sent out the materials in august and i had it finished by christmas which left me more time for the modules i was starting in february. the ones i'm doing this year are going to be more time consuming.
i already have a science degree and I'm a teacher so i'm not finding the maths so far too difficult and of course i've a lot of time in the summer to work at it, but there are plenty doing it with regular 9 to 5 jobs that don't have as much time off. I was always interested in maths but never got to study it and i picked the maths and stats over their other maths degrees because it doesn't have a summer school (available in UK only, so involves travelling) and it's recognised here for teaching purposes.
cost - £755 per 30 credit module, so it is expensive, but that includes everything. as sterling is weakening at the moment my fees for next year are going to work out a lot cheaper than last year. at today's exchange rates a 30 credit module is working out around €810.
PM if you want any more information.