You use "そろそろ" when you want to say "it's about time" to do something. For example, if you are going to see a movie at 7:00 p.m., and it takes 15 minutes to get to the theater, at around 6:20 or earlier, you might say "it's about time to go" (or, そろそろ行きましょう）

Brian is totally right!!