I can't imagine having another now (DD is 17.5 mos and likely an only). We co-sleep (well, DH co-sleeps with DD...they're too "active" for me to sleep with...I sleep on the floor - but, that would still leave me sleeping with one of them...ye gads!!!!!) and I doubt she'll give that up anytime soon. She's jealous...of me and DH, of DH and the dog, of me and anything other than her. Yeah, it might be different with different kids, but, I'd have to say 3-4 years is probably good...I was 31mos when my sister was born, and I was old enough to understand sis needed mom, and old enough to amuse myself, and I even went on vacation with grandma shortly after....I can't imagine doing it with an 18-24mos old. We were still close enough to play growing up, but far apart enough to not have so much competition...on a practical side, I'm still pumping for DD, and would like to make it to 24 mos.
Everyone I know with a kid DD's age seems to be having number 2, but, there's no way in heck I could do it...she's finally becoming a "human" as in understanding me, being a separate person, etc. If we were ever to have another, it wouldn't be until DD was *fully* out of baby mode (ie...4-5 years)....
If there were a time limit, that would be diff. to me (my family actually has a HORRIBLE repro. history with medical hysterectomies at 27-39 in the prior three generations, but, we're looking at an only, so not a factor).
I also agree with the help factor....my parents lived with my grandma until I was seven, so, that was a factor. My DH is military, and works 12-15 hour days, so, yeah...it's hard sometimes with just one...I'm sure if we were closer to family, it could make a difference.
Oooh..forgot to add...my DD is in the process of giving up her ONLY nap (half the time, yes, half the time, no)...that has a *big* effect on me.