Gravid spots aren't a good way to tell. I have had female guppies who have always had a dark gravid spot even when they have never been bred before (raised in an only female tank) and I have had some where they don't get any "gravid spot". But looking at your female She doesn't seem very long or very old. Her tummy isn't super huge, but she is also a smaller female.
Because she is getting a bit agressive I would say maybe a week or two max. It is hard to tell with a picture.
I hope this helps, sorry if I am off!