It really varies, some dogs go into heat as early as 4 months (very rare) and some as late as 18 months. My first dog went into heat at 6 months, her sister at 8 months. You will probably notice the blood first, but you may notice that she licks a lot.
Some dogs have silent heats (also rare) where you have no sign at all.
There will also be swelling in the vulva and sometimes they get swollen nipples.
If you want to keep the males away, don't use this time to take your girl for a walk...she will increase her urine output and this is a signal to males...so they come from all over the place and follow her trail. If you keep her home, likely, you won't have unwanted male dogs hanging around.
Heat cycles last anywhere from 4 days to 3 weeks...my early girl is in heat for about 2 and a half weeks, her sister usually only a week and a half.
The panties work well to keep blood off of things, but can cause infection and certainly don't protect her from pregnancy.
Thanks for the reply. I think our little girl is growing up. I came home today to find a little mess on the fouton in her favourite spot. Also, her male friend, whom she plays with regularily, was paying alot of attention to her down below, something he has never done before. there was even one mounting attempt. Time to keep her away form the boys for a while. Thanks again Jac, you are always very helpfull. I appreciate all the info you contribute to this forum.