I haven't played in at least 4 years, probably more, is the standard opening still $12-$15 in the 1=2 game? Or have the games tightened up as far as the initial opening raise? How about 2-5? $20 opening still routinely getting 2-3 callers?

Depends on where you are playing..

Sometimes, it depends on whether you are at table 1 or table 7 in the same room.

Depends on the time of day also.

Years ago, I almost never played on a 1-2 table, in the Northeast- MS, Foxwoods and AC, that the opener didn't come in for at least $12. And I probably played ~ 4000 hours between 2006 and 2010. Is that still the case today? Or do you see a lot of tables where the opener comes in for $6 to $8?

Depends on the game. I may have 4 tables of 1/2 going and at one table a $12 open may regularly not get any callers and at another table $12 will get 8 callers and a $30 open will get 4 callers. As an average though I would say $10-15 open raise will get 2-5 callers.

Where I play 1/2 in northern California this is the case also.

I've occasionally played at 1-2 tables in the Midwest where the standard open was as low as $4-6.

You still have to be able to read the table...others have pointed out common occurrences, but I have also seen tables where it limps in eight places and even a raise to $20 from the blind will get called in at least six places, with at least two of the last three uttering something about "well, the math practically works for any two cards at this point."

Next day could well see many of those same players folding to a small $5 open...

Yes Not really, but some players raise to these amounts usually with marginal hands for different reasons.

It's funny because this reminds me of my early game with all the OMC regs and there are at least 4 of them that you can tell exactly what they have when they raise. $6-8 is AK. $10-12 is JJ or QQ. $14 is always AA or KK. They never raise with any other hands. You can call this with 100% certainty every time.

Welcome back Spaceman .. hope you run well to get your feet wet once again.

The posts pretty much cover it .. day game $6-10 .. night game $8-13, both with anywhere from 1-5 callers. I think you will find players just as loose, but perhaps more skilled post-Flop than years gone by.

As an overly active player I open my whole range with $7, $11 or $16 with 'a little' regard to position and limpers ... But I actually put a little thought into the rake as well.

$7 ... 5 callers plus a blind .. rake = $3 on 36/37
$11 ... 2 callers plus blinds .. rake = $3 on 36 ... 3 callers plus a blind $4 on 45/46
$16 ... 1 caller plus blinds .. rake = $3 on 36

If I take it down with a c-bet then I get max pot on $3 rake. If not, then we are fully raked and go from there. A lot of rooms 'round up' at 37 or 38 for the fourth $1 so these are my 'spots' that come up often. Of course an extra limper or two messes things up ...

A while ago in one room I was 'accused' of being staked by the room to make sure that all the pots get 'a lot' of rake since I rarely limp and play lots of pots. GL

The above post regarding rake/drop considerations is spot on. California games are raked at a set amount, by law, and I tend to see a bit higher opening bets- in an effort by some to cover a drop from $4-6, from NorCal down to LA, location dependent.
Nevada, either LV or Reno, I see a bit less average open since those takes are a % take.