Decibel readings are extremely affected by distance as has been mentioned. I have a meter hanging on my wall (just for fun) and with plugs in I've seen mine go over 120db with a 2203 before I even got to "5" on the master volume. If I held it right up against the speaker it'd probably hit 130 at the same volume level. It's also worth mentioning both my cheap-o amazon decibel meter and the phone app meters are notoriously inaccurate, +/- 10% or more the actual reading, and there's also a difference between measuring DB "A" "B" or "C." To avoid boring details, DB "A" is closer to the human ear, which perceives lower frequencies as being a little less loud, while DB "C" is a flatter curve (technically more accurate, and also a more expensive tool).
A huge factor I haven't seen mentioned yet is speakers. 4x Vintage 30's are a lot louder than 4x T75's for example. You can tell quickly which speaker is louder at the same input level from the amp by looking at the "sensitivity" spec - even at the same frequencies (of course, perceived loudness will be more with brighter speakers as well)
Wattage of the amp has a lot less to do with it than people might think, 50w for example is only 3db quieter than 100w. However it's also worth mentioning that different amp makers measure output wattage differently - ex. a Marshall JCM800 2203 probably measures closer to 150-160w if you play it with both preamp and master on "10."
No idea how far you could hear it though. Depending on where you live I think it'd be pretty funny to have a friend go a neighborhood or two over, check the distance on google maps or something and see just how far it goes. Just don't play for long or the cops will definitely show up if you're in any kind of town/city. You should do it and report back!
Happy 4th!