It depends on wether or not you do the learners. I have a 'blank plan' that says 89 days without or 57 with learning skills, a bit less if you optimize your attributes. Can easily be done in 50 days if you we're willing to cut corners (I include Drones V and Warp Drive Operation III in my plan).

Edit: I think it takes about 3-4 million skill points for Exhumers? (This is a guess...) So that would cut the 50 days down a lot more, possibly to just over 30 days. With a buddy trial from your 'main' you'll get 51 days 'free' plus 20 Euros to transfer a character. Thus making a decent mining alt is only down to the transfer cost. Pretty cheap!