It takes about a minute or so to be fully warm if your tubes are relatively fresh. Any longer will indicate that your tubes are getting worn or that your tube heater circuit needs attention though this may also indicate that your voltage isn't up to snuff.
First I would tune the guitar. Then provided your amp is plugged in, I would plug a speaker cable into the output jack on the back of your amp and input jack of the speaker cab or check your cabling and impedance, plug in your instrument cable into both guitar and amp, turn your amp on in standby, prepare your playing area (location of effects, battery checks, location of mics for cabinet and vocals- if any, check your midi footswitch programming- if any, etc.), strap into your guitar, check to make sure your volume and tone knobs are spinning smoothly then place your tone knob where you like it and zero your guitar volume. Then with pick in hand, take your amp out of standby. Roll your guitar volume up to where you want it and play something arbitrarily (an open chord is good for this)to ensure that everything is functioning while slowly bringing your amp volume (master if so equipped) up to a good working volume for where you are playing unless you have already soundchecked for the venue then just use the guitar volume to bring the amp up slowly and leave the amp volume where it is.
This process should be pretty reliable for having your whole rig setup and running properly and will not really annoy anyone in the process. If you are not going to play immediately continue your sound check and then leave the amp in standby if you will play within an hour or so. If your slot is later then maybe power to standby between sets if you intend to leave your rig on stage. Your actual setup procedure may vary depending upon the level at which you can leave your rig setup between sets. Obviously if you open you can do whatever you need to then just leave your amp in standby from a few minutes til showtime. Some guys leave their rig on from just before doors if they open. I would suggest powering to standby while the prior act is tearing down provided the stage supports it if you are not the opener.
If you are just playing at home with everything already setup then power to standby after: double checking your speaker cable connections, impedance, and instrument cabling. Once in standby: Check your batteries, power supplies, midi switching, etc., strap in unless you intend to remain seated, check your guitar settings and knob functions, get your pick ready, bring your amp out of standby slowly with your guitar volume knob.
The reason that I suggest bringing your amp up slowly even with warmed tubes is that it is less of a shock to your tubes for an initial use. It may or may not have any real consequence if you don't but it has never hurt and this is the way I have always brought an amp up and out of standby.
I have only ever had one issue and that was because the person I was teching for at the time had some pretty worn tubes and they actually failed. It was just a simple preamp tube but we just ran a backup for the show and everything was fine. The next day while we were troubleshooting the amp, I found the faulty preamp tube and everything was fine after replacing it.
Other than that one incident I have never had a tranny blow or tubes fail from incorrect loading, effects incorrectly routed, batteries fail during a show, power issues, amps catch fire, awkward body positioning to use mics set at wrong height for the user, bad pa sound due to incorrectly aimed mics, etc. This is not to say things don't happen like tripped on cords unplugging or mic stands getting bashed then having to reposition a mic. I have even had an amp topple and managed to restack it while the show went on.
I guess all that wasn't necessary but thought I would back up the thought of a minute or two minimum wait time in standby. I have been caught taking an amp out of standby though at home within 30 seconds though. Sometimes you have something in your head and have to play it immediately or you lose it. I wouldn't suggest taking your amp out of standby with this short of a warmup regularly but if you absolutely must try not to make a habit of it.
I have heard my own tubes sound better with longer wait times but like I said it was usually when the tubes were pretty worn or on their way out.
