YMMV but this is what it was like for me.
I started at 40mg but my follow up appointment I went up to 60mg.
It started working immediately after the first dosage.
I'd recommend starting in on a day off.
It was intense at first but amped down over time as I got used to it. Basically before it was like my thoughts were a 4-way stop with no lights and everyone was going at the same time. At first it was like a metered on ramp, but then it became like a 4-way intersection with normal stop lights.
Basically for the first few days it was like I had developed a super power. The power to start a task and finish it.
In terms of side effects I had a few at first that really sucked.
I had gnarly dry mouth for the first 2 weeks. I felt quick to anger for the first 2 days both when I started and when I went up in dose. My ability to fall asleep went down for the first month, so a very small amount of melatonin was pretty necessary.
NSFW side effects: Erections were a bit different, basically random ones while I was feeling the effects of the drug were mostly gone but after it wore off were more common. Also waking up in the night with one was basically a twice a night thing. Everything is mostly back to normal on this front after a few months though.
Edit: Also be sure to eat with it, the first time I didn't I got so nauseous I was shaking and sweating. Literally ruined my entire day.