Minimum of 70 days but most people go over this, some by a considerable amount due to not having managed all the trips for routes, etc. I know you're obviously keen to know what lies ahead but my advice is to just assume it will take the time it takes. During your time with your DI they will take annual leave, you may also do this so combined that could 4 × 2weeks that you're not driving for due to the DI or yourself being on A/L. You will have a couple of weeks back at the training school doing risk/rules courses, you'll have nightshifts where you're shunting sets about the depot and won't be counted as a day of driving and no hours clocked up as you're not out on the mainline. You could also be sharing a DI with another trainee so only get half the hours driving for that day. Lots of things make it take longer than just 18 weeks with a DI (18 weeks × 4 day week = 72 days driving) Your money goes up the moment you get your licence as that's you out on your own but it goes up in increments as stated in the job advert you applied for.
The first 3 months is slightly harder for some than others. Some breeze through and naturally absorb information, some need to look over notes in the evening when travelling home or when in their hotel room. Some people find the rules easy but the traction difficult as they don't have a mechanical mind and find it daunting. Once they realise it's not anything scary they get on fine.
Don't worry about it, enjoy your time as a trainee and read what your trainer tells you but nothing else. Trying to read ahead leads to problems with misunderstanding, do as your told, follow instruction and you'll fly through with no problems.
Congratulations on getting the job, sit back, relax and look forward to it.