I think with Dermatica I started to see a glow about 3 months in, but it's not until 6 months you really see the difference. It's long term skincare - I plan on using for life, and it's so effective that I've actually saved money by cutting back on other, fancier products.
In answer the the question about cost - its a £20 a month subscription, BUT you can choose to have it delivered every other month, or every three. Tbh I think every other month is enough - you only need a tiny amount, and especially at the start you will want to take it slow as it can cause irritation if you dive in too quick. So you're looking at £10 a month.
I've never finished a bottle before the next one has arrived and I do every 2 months, so reckon its enough!
A few other things. They will usually start you on a very low dose and then you gradually work your way up as your skin gets used to it. You will start on 0.25% and then if you think after a few months your skin can take it and is no longer feeling irritated, ask them to move up.
I've been using it 18 months and am now on the highest prescription of 0.1% which is the strongest you can get. No need to rush though- its slow and steady wins the race! Rather than getting a facial or treatment that shows instant but short term results, think of tretinoin in terms of years. No rush, no stress, and no problem if you don't use it every day/ have a little time off.
I tend to use mine every other night, and don't take it with me on holiday (it makes your skin more sun sensitive so a high spf EVERY day, factor 30 or more) is needed.
Unless you have the skin of a rhino, expect it to be sore at the start, at least the first few weeks. You can help ease that by: not using too much, using it only 2 or 3 nights a week to start, and "buffering" it with a plain moisturiser afterwards. Also let your skin properly dry after washing it before you apply as it sinks in quicker into wet skin and can irritate more.
If you find your skin is sore/peeling, just take some time off, use lots of ceramide and hyralonuic acid cream (Cerave moisturiser is perfect for this) to help repair your skin barrier, and when it's calmed down start again.
Your skin will get accustomed to it though, it just takes time and its different for everyone.
Also, if you're prone to spots you might find you get some zingers at the start! I certainly did - a few really deep painful spots that came up, didn't come to a head, then went away. But eventually the spots stop :)
There's loads of advice online about how to ease into using tret.
Anyway, as you can tell I love the stuff! My skincare routine is pretty simple- a good cleanser with hot cloth, tret and cerave at night. And then cleanse, maybe the occasional glycolic acid toner in the morning, a simple moisturiser and good spf. I do use other things occasionally but mainly for fun/added extras! Tret is a powerful active so I advise against using vitamin c or any acids with it at all until you're a few months in and your skin is used to it, or you're taking a break from tret to try something else.