How long can I expect my painted metal door to resist rust?
TLDR: 1 and nope.  

Paint snob here.  My life's goal is to never paint something twice.  I think nothing of spending $300/gal. on aircraft grade paint.  I've evaluated a lot of the work in the field, including NASA's acclaimed book Hell of a place to put a million tons of military-critical steel latticework, seriously, we couldn't have done this in Arizona? now a website at http://corrosion.ksc.nasa.gov.  Which has some interesting stuff, mostly the idea to take paint seriously. 

It's mostly in the prep

If you knock it down to SSPC-SP10 (sandblasting), even a poor primer will perform quite well because there isn't any rust. I bought a ton of $10/gal primer from a paint company right after the lead ban (when paint wasn't so great), and it's lasted 30 years on a sandblasted surface. 

You're doing more like SP2 with the sander (they're categories, it's not linear) and you plan to do SP-nothin' on the next door.  By the way, that's gonna look ugly as sin.  It will be obvious to the buyer that you just painted over rust.  

Keep in mind the rust is acting under the paint, and one more paint layer won't do anything at all, even if it's the aircraft stuff.  Needless to say I don't waste any aircraft paint on poor prep like that (even when necessity forces me to do it). 

Water content accelerates rust. When I must paint over any rust, I prefer to do it in direct sun in low humidity days with the surface temp nice and hot at 130-140F. That removes as much moisture as possible from the pores of the rust. 

Paint matters too

Especially if prep isn't fantastic.  I've found over rust, the exotic $100/gal primers don't work... rust converters don't work at all (NASA backs this up)... ironically what works is plain ole cheap Rustoleum 7769 Rusty Metal Primer.  Yes I know it's brown.  If you want a white one, I don't know what to tell you.  I haven't found anything that works. **  What I do is wait for the 7769 to dry fully then top it with 7780 Clean Metal Primer just for the white coverage. 

So if it were me, I'd go back and wirebrush and then sand each exterior door til you've removed as much loose rust as possible, 7769, then whatever other paint you want from there. 

Don't leave it in primer

Primer is not paint.  It is not a protective coating.  It is designed to prepare the surface for a protective coating.  You need to follow up and put a compatible topcoat on top of the primer. 

If you need evidence of this, just look at any old car driving around that has been laboriously sanded, bondoed and primered, and now has rust eating through the primer   because the owner couldn't get paint money together. That is good primer on a well-prepared surface, that is failing because primer isn't paint. 



** in fact, I intentionally use white primer on surfaces I'm unwilling to fully sandblast -- let it sit a few months (yes, in primer) and rust punches through wherever it will, easily seen against the white.  Sandblast those spots and done.