this is an interesting thread to me- I would say 
that the average time spent collecting is 
probably 2 years- most people run out of money or 
enthusiasm.  It takes such a long time to build a 
really good collection unless you have a lot of 
free money and can purchase overpriced cels that 
would normally sit on a dealer site, or offer 
enough to a private collector to let go of a cel 
he/she normally would not sell.
I also think the updates have not been as good 
and that some people are trying to overcharge for 
not-very-good cels, but I have seen some very 
good cels not selling well either.  It would be 
nice to have some more updates of some nicer 
cels, but not at 2 to 3 times the price they 
would usually go for.  
Cels which I have been enjoying that do seem to 
be getting more reasonable are Ah My Goddess - I 
still really enjoy FY bishonen, but they tend to 
be few and far between and overpriced.  I'd like 
a nice AnC Yuuhi too, but I'm still waiting 
for "the one" and I'm hoping to spend about $50 - 
$75- with prices being low on AnC, I guess we'll 
just see.
I think I keep collecting because #1 - I'm 
addicted, #2 I still really love the art, and #3 
Hope springs eternal... I may yet get that 
tamahome I always wanted (though it's not like 
the half dozen I have are not wonderful), or that 
Juri movie cel, or.... you get the idea.  :)