My GSD bitch came into season at 9 months,  she is 13 months now and I noticed she was in season again 2 weeks ago, which took me by surprise as I was not expecting it till at least  the end of march, she has bled much more this time and leaves a mess when she sits down, she is very good at cleaning herself, as she was bleeding quite a bit when i first noticed she was in season can any one  say how much longer she has to go, I intend  to have her spayed in 3 months time. thanks

My second shep had seasons every 3 months or so and I lasted through a few, but she also had phantom pregnancies so it was somewhat of a nightmare.  Personally I believe it's better for their health if you can last a bit longer, but 16 months isn't too bad.  Try and time it to mid season if you can.  Seasons last around 3 weeks.

4 monthly seasons seem quite common with GSD's so you may be better to wait for the third to see if this is a one off short interval or how they are likely to be. 

If it's 4 months again then you will need to time spaying for 2 months after the start of the season, as you so not want to spay shortly before a season due to the higher risk of bleeding with the increased blood supply to the reproductive organs as they are getting ready to come into season..