Old work electrical boxes

I've added 100 boxes and was done in 2 hours, including primer mudding the box tight with joint compound and fiberglass tape. 
Next day pain touchup..

A 2 x 4 wall rip a 2 x 4 to 2 1/2 inches and the width of the studs 14 1/2 usually. Check after the hole id cut.

Cut the hole for a regular single or double and slip the cut down 2 x 4s into the opening horizontally.

You need to fit the width before the liquid nails.

Put a generous supply of liquid nails on both ends. It may be a bit messy, thats fine. Secure them in place with a few screws through the sheetrock.

They are narrow enough to flip in the hole you cut. The wires will fit behind the cut down 2 X 4s

Secure the box, pull the cable (more than one wire), 

Your ready for mud. Tape, Top, Skim, and texture. Can skip the last two for a smooth finish and sand after a thick Top coat.. You want to finish it all in one day.. Hot mud.. Add a hardener.

You'll be at primer in 2 hours.. You can rough out the electrical and hook up add a cover and flip the breaker. The next day you could touch up the paint. BUT use it in 2 -3 hours..