Slowly.
Oh, I got a few of the toys when they first came out, including Bluestreak as a Christmas present, but it was just one of many things that caught my interest.
One of my roommates in college was really into it, though, so I read the entire G1 comic (to that point) from his collection and started to get interested again.

But it really took off thanks to the Legion of Net.Heroes.
Several of the other writers in the LNH (on Usenet, originally rec.arts.comics, later alt.comics.lnh and rec.arts.comics.creative) were into Transformers, and that spurred my revived interest as well.

And once alt.toys.transformers got made, I ended up heavily into the online community and that turned the interest from casual to committed.

It comes down to how horribly disappointing BotCon '96 was.
3H wanted to do a lot of special stuff to make it up to the fans.
By that point, I'd been making decoys for a couple years, slowly (I showed my first few at BotCon '95).

3H thought the decoys would make for a cool "by fans for fans" dinner exclusive, and in late 1995 asked me if I could manage a hundred of them in time for the convention.

They supplied me with a bunch of decoys (and let me trade out my own doubles for ones I didn't have) and I spent months painting them.
I wasn't involved in the presentation elements like the base or the cards, and in fact first heard about those parts when people at the con posted about it. :)

Oh, I have no idea.
A lot of the time was while watching TV, so it was "spare" time anyway (I had no computer at my apartment back then, so when I was home there weren't many distractions).
I'd pretty much finished the coursework part of my grad school time, so I didn't have a lot of work to bring home with me from the office.

All in all, it was a sort of golden time for me to work on art.
I tended to work in groups of three to six at a time, assembly-line fashion (especially subgroups where they'd all use the same paints), so it's hard to say how much time was spent on any one figure.

They sent me two of the Japanese gift sets (the big one and one of the medium ones) and a lot of loose decoys.
Plus a few extras just in case something got ruined.

And as mentioned earlier, I got to swap out some of my doubles for ones I didn't have (like Ravage or Laserbeak). I think they sent the decoys in two loads.

Nope.
Part of why I agreed to do this was that I wasn't able to attend (it was outside of driving range, and as a poor grad student I couldn't justify airplane tickets on top of all the other costs... and I think that may have been the year I dropped a quarter of my pay on dental work).
So I was there in spirit.

Very positive.
They asked me to do another small batch for a board game at BotCon 98, in fact.

Transformers, various superhero toys (especially Iron Man), Lego and similar lines.
A lot of it is "whatever looks nifty at the time".
I'd say I value Transformers over the rest, though. I'm certainly less likely to have ditched or donated Transformers than other toys while cleaning out for the move!