I am wanting to build a mountain surfer and having a hard time wrapping my head around motor sizing.  Most of the locals here just fly thier 5" freestyle quads with big lipos, which I "could" do but want somehting a little more purpose built.  So my question is how can I calculate estimated flight time?  I have an Apex mini 4" As it sits its 340grams AUW with an 1100mah Lihv and RCinpower 2105plus 3600kv motors.  Its a freaking rocket! but only get like 2-3 minutes of flight time.  

What I was thinking...First add a SuperD diversity reciever then get a set of 5" arms for it.  I have an abundance of lower pitch S3, P3.3, and S4 props to use, and was thinking to motor limiting the motors to like 2500kv? in betaflight.  Weight should be close to the same and flight time should increase significantly.  Do you all think I could get 8-10 minutes with this strategy?

i believe in case of an apex, its not tooo lightweight.
lower pitched rotors doesnt need to be more efficiency. higher pitch on low rpms could be more efficiency, maybe. high pitched biblades. sadly there are none

use ecalc.ch, it can bring you to the best combination.

10minuts wont be a problem, i beleive even if your apes is fully loaded: good matching rotor and battery should bring you 15min flighttime, more than 5k total flightdistance.
ecalc will now that better than me :-)

I had a nice 30 minute discussion last night with Chat GBT lol. Pretty much spot on what you said. I always forget about calc but the free version actually had the motors I wanted to test and also confirmed that on 4” props at 1850kv I “should get 10 minutes hover time. On 5” props that goes up to 15! And like you said, prop pitch did not make that much of a difference as long as I stayed within 2.5-3.4. I will be doing a test flight with lowered kv today cruising around the house with 4” props and will see how it feels before ordering 5” arms.

You won't know till you try! But having shorter arms is going to hurt flight time too, that's why a lot of longer range builds are 6-7 inches frames with wider stator motors and lower KV for efficiency. But that's why the Apex is such a great frame... because it's extremely versatile and comes in sizes 4-7!

But I doubt you'd get 8 minutes of flight time. My guess is closer to 5-6 if you're lucky.

So the results are in and I am pretty pleased to say the least.  I went ahead and got 5" arms for my micro Apex. I run Ethix Mango 5.1 x 3.3 props on my freestyle builds so thats what this build got.  Motors are still the RCinpower 2105plus 3600kv, in betaflight I scaled the motor to 63% so they operate at 2300kv.  And.... so I could break the $100 barrier for free shipping from Pyro went ahead and picked up a SuperG Gemini transmiter and SuperD diversity reciever.  AUW with a 4s 1100mah Lipo is 373g

Its insane but it is whisper quiet!  I mean its quieter than my 3" I am currently using default Betaflight PID's and filter settings and it flies butter smooth.  It has enough punch to manuever quickly quickly get over a tree if needed.  but best of all, I flew down to 3.55 volts before I stopped.   I bet I could have pushed it probably another minute but got 8 minutes 40 seconds flight time