As she gets closing to giving birth, the gravid spot will become much darker, and she will look triangular when looking from the top or the back of her. When she is ready to give birth (within a day or two), she will start to hide in any plants, wood, or anything large and become very terrotorial when other fish try to get to close.
If you have another tank you would be better off removing her into that tank or put her into a breeding trap a couple of days b4 she is due to drop to prevent her from being attacked in her most vonerable time and to also prevent other fish from eating the fry
Just keep in mind, females will always have a gravid spot, as that just shows they are female. Just because they have one, doesn't mean they are pregnant.
guppies take about 30 days .you should put the female into floating tank. as the other fish will eat the fry and so will the female. the floating tank has a sperater so that the fry can escape to the bottom out of harms way. it is best to have a small tank to put the fry into. this will give them the best chance of survival i am pretty new to breeding guppies but on my 7th batch of fry. they have normaly between 5 and 15 but can have more so extra room is needed for them .GOOD LUCK with your fry hope you get to enjoy the fry