Warm up time for record playing

Others have stated that it’s the time your ears/whatever take to adjust. Makes sense on a certain level, and I agree your ears probably do need some time to adjust to volume levels being projected at you in your listening room vs what your ears have been processing throughout the day prior. BUT, I have the same experience with my analog rig in regards to warm up period. I will put a record on and leave the room and come back only to flip sides. After a couple LPs I come back and start really listening. I can hear an improvement vs when I start listening without a warmup. I wasn’t there listening throughout the whole warmup period so that rules out the theory of my ears adjusting. It’s cartridge suspension, TT motor, whatever it is, is warming up. That’s been my experience. 

Yup, it's the cartridge suspension alright.
Agree one LP side is about right.

Back in the day we used to aim a tungsten bulbed desk light with a reflector just above the cart to warm it up quicker.  Can't get those any more.

Rvpiano, as I mentioned, your cartridge’s suspension is probably relaxing during the first half hour or so of use. However, as tooblue and others have said “a little of this and a little of that” is probably closer to the truth. For example, how is it possible for “the rest of the system is (to be) fully warmed up” if one has not listened to LP’s yet? Whether stand alone or integrated a phono stage needs to warm up as well; especially if it is a tube unit, in my experience. My experience has also been that it is not enough to simply turn electronics on and let them idle for a while. One has to actually play music through the system for it to be “fully warmed up”. In the case of my all tube system, and as silly as it is to try and quantify these things, I would say that it sounds about 75% of the way to reaching “full sonic potential” after about one record side and pretty close to full potential after two record sides. Full potential even later than that.  

Gosh Millercarbon is spot on for a change!
I have a spotlight too as well as hair drier for the cartridge
And my tech guy is building a bespoke phonowarmer so I can plug into  the phonostage and "tickle" it until all bits are warmed up. Saves cartridge wear and getting up and down changing lps just for the sake of warm up period.
45 minutes is about right for me too for all parts of the chain to gel together. First unwarm period - meh ... 
Warmed up - Yeah

In my research for a new cartridge I came across info which I dont recall if it was people who owned Lyra cartridges or John Carr himself but I do remember it being said that if your cartridge is in an environment that is less than 72 degrees you should use a light bulb over the table to warm up the cartridge.
Reading this I started to pay attention to the sound when I play my first record compared to one’s say a half hour in, and I gotta say this is absolutely true.
Now when I fire up my rig, I give my tube gear 20-30 min warm up then I play one side of an album with the volume off to give the motor an cart. some warm up time.