Hi! I've always been told that follicles over 12mm grow between 1.5-2mm a day so my guess is you'd trigger in a few days if that's your only follicle. Mine usually grow about 2mm day but sometimes more and sometimes less. Are you doing a natural ivf or stims? That might also affect the growth rate. This cycle I had a 13mm follicle on a Monday and on wed it was 19.5mm with no meds so thought that was interesting...i didn't have my fsh tested but maybe it was high and why it crew a bit faster than normal?
Good luck with everything!