It's different for everyone, but you used the word in your title - progress. The disease is progressive, so the odds that things will stay exactly as they are now are pretty much zip. People go into remissions, some seem to be relatively stable for a while, but in the end, it always progresses. The fact that we actually don't know how fast it's going to progress, or how severely, is the main reason to make sure it's treated with a DMARD (conventional or biologic) as soon as possible. It's not always nice to hear at the start, but there's no point waiting and in two years time going "oops, it did progress quickly...."
My dad had very mild PsA (I'm pretty sure that's what it was), with just a tiny bit of psoriasis in his hair. It took them about 20 years to offer him a DMARD, and by then he'd given up so he didn't take it. He got PsA initially when he was about 26, just in his lower back at the start. It never did anything spectacular (hence the delay in any form of diagnosis or treatment), and it was common for him to go for 3-5 years at a time with what could be described as "stable" disease, then have a bad 6 months, repeat cycle. By the time he was 55 he couldn't dress himself because he couldn't lift his right shoulder at all, and needed a stairlift to leave the house. It only progressed very slowly, but the rest of your life is usually a really long time, so even slow progress can add up to a lot.....
Having said all that, once you get a DMARD/biologic that works for you, then you know you really are slowing down the progression by a huge amount - some people get many years of remission or near remission without much in the way of joint damage.
It's a really difficult time, facing that feeling that things are pretty darn awful now and they are just going to keep deteriorating... but funnily enough, if you ask they guys who have been around for a while, they'll tell you that's not actually what happens. Sure, you might have some more troubles with your joints, and maybe even a few other PsA things (like eyes for example), but as you come to accept the diagnosis and learn to live a great life as the new, reinvented you, you actually feel like things get better. And I can certainly say that finding a treatment that works, and reduces all that extra TNF running around in your body (which has been linked to depression, cardiovascular disease - the list goes on) helps the process enormously.
As a very odd aside, I'm going to add that for the first time in years, I felt frivolous and healthy enough to buy and wear some beautiful italian leather high-heeled boots! It sure hurt my feet, but it helped my soul :)