there is more to to take in to consideration, like age, size etc..
The best thing to do is, when they have had thier first drop, write down the date, they should drop again 4 weeks after (6 at most but very rare), you will start to see a patter in thier drops and this gives you a more accurate time of when they will next drop...
Otherwise, there is other signs..
They will become very unsocial
The anal area will slighty open up
The back end of the fish's belly will drop as much as the front causing it to square off.
The gravid spot may darken (not the best indication)
You will see little black dots in the gravid spot (the fry's eyes)
From a over head view, she will look very wide.
From a back or front view, she will be ball shaped.
You must take in to consideration that smaller fish will not get as big as adult ones when pregnant as they produce less fry
Other behaviour when ready to drop...
The fish will be static in movement whilst dropping fry, (only while the fry is actually coming out)
They may hide away in plants and other areas.
They will dwell on the bottom of the tank.
They will go to corners of the tank (top and bottom)
They may hang around the heater, internal filters, etc.
Read my pinned thread at the top of this section..
click here