It took me a good 6 months, and after a year I’m reaching 2’6-3ft. That was with a lot of practice during the week though.
I bought a new horse 6 months ago (11yo, QH gelding) and we are at 2'. He is def green but he wasn't responding to cross-rails and so we raised it a bit for inspiration a small bit at a time and are now at 2' and he's responding well to our once a week schools.
I've been riding for 10+ years on other h/j horses so many miles in the saddle. PATIENCE is key and as Kandy Fox mentioned - it depends... there is SO much more you can do vs jumping into jumping...pole work for one can be a fun was to learn the key skills needed for jumping.
i've been jumping for 2 years and just started to reach 2 ft :/ it depends on your horse, your trainer, and your determination.