I was wondering, specifically for small dogs and/or hounds:

- when is the normal time (for example, "around 6 months old") for a bitch to go into heat her first time?
- how often do most bitches go into heat?
- is it common for heat cycles to shorten from X months apart to Y (a shorter timespan) months apart over the course of a bitch's life?

Dogs are a bit like humans  it just depends on them when they come in season.  First ones are usually 6 months to around 9 months.  They are supposed to have a season every roughly every 6 months.  Even if your bitch is normally regular they can sometimes do a funny.  My youngest girl was due in season December and finally had a season this week!  :)  Now my friend who has litter sisters to both of my younger girls always has hers come in season on average two weeks before mine. 

A bitch's first season can be any time between 5 months and 12 months - with extreme examples even later than that - though the average is about 6 months, and it's the same whatever the size of breed. My medium-sized bitches have generally had their first season at about 8 months and continued with seasons at roughly the same frequency throughout their lives.

If you get a copy of 'The Book of the Bitch' by Evans and McKay it'll answer all your questions.
:)

Thomas and Jean,

Thanks so much for your replies, and I apologize for how long it's taken me to do the same.

I was mostly curious because of my family's and extended family's dachshunds.  My eldest and her cousin came into their first heats at almost nine months, and I was wondering if it might shorten up.  Turns out, both of them just started their second heats this week and last week, respectively, so they shortened to six months.  I guess I'll see if it stays even like that.  :)

I'll definitely get my hands on a copy of _The_Book_of_the_Bitch_.  Sounds interesting, and I'm always ready to know more.