Gonna say 2 years, maybe 2.5 yrs. The problem, if you wanna call it that, is that everyone is trying to figure out the new girlfriend on day one and that just isn't possible. The last 18 pages are replete with speculations of "she's only doing this to take advantage of him" or "she's fine, she knows what she's getting into." As much as I love autistic speculating, we aren't going to have the true story on her and this new saga for a while -- personally I think it'll be at least a year before we get the full heading on her real motives for getting with him, and if she really does know who and what he is, or if she got duped like Leanna did.
iirc when Leanna first came to town, she had no job prospects and didn't work until much later which was probably a huge strain on da money. Seeing as she appears to actually be financially independent (though not especially wealthy) it may be that her financial support and "street smarts" when it comes to money end up benefiting DSP and making his "legacy" semi-sustainable.
He's not that far off either. He just has to keep a decent amount of Twitch subs and then he's on easy money street.
My bet is that this new character in the DSP show may actually be a benefit to him in the long term, and they may actually just genuinely "get" each other. A lot of people seem to think every relationship in life is about the storybook wedding/romance because they've watched too many movies. The modern idea of romance and courtship is only really talking about recent history because photos didn't have widespread existence until like 100 years ago.
Not every couple is the type you'd picture seated in a picturesque Condé Nast Traveller getaway enjoying a plate of fine cheeses and merrily pouring each other a glass of vintage wine while the fireplace roars and the kids play peacefully in the background.
Kat doesn't seem super social and I've met plenty of couples where either one or both participants were not super social people. It doesn't make them bad people, it doesn't mean their relationship won't work out and it arguably doesn't matter so long as they're happy.
"How long will she last?" is an interesting question, however again, look at the current idea of relationships and the historical context behind them. This idea that you're going to find your soulmate, settle down and live together until you die isn't very realistic the reasons that relationships were so long lasting (such as economics, religion, having children, morality and your Bushido honor code) aren't as prevalent any more.
If Phil and Kat have a loving relationship that lasts a decade or more then they're actually an exception to the rule. Because, really, most people do not last nearly that long nowadays.
It'd help if you looked at things in terms of life in general. A lot of people seem to think that its the end of the world because of all the shit that's happening. The truth is the human race has never had it easier or better than they have today, which means its a hell of a lot easier to move on from a relationship you're bored of. I can sure as shit assure you that no one who is reading this thread likely had parents who still went out in the fields to pick vegetables to make sure they didn't go hungry.