Yeah, too hard to say. It's like saying, how long would it take me to be able to run a half marathon. Too difficult to spell out.
For me, when I first started, it took me probably a couple months of riding only on the weekend to be able to ride 25 miles. I wasn't in bad shape, but I wasn't in shape either. The most difficult was adjusting to the saddle and my stamina with the cardio.
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Demented internet tail wagging imbicile.