Some of my paintings are small studies 6"x8" t0 11"x15", completed on location (plein air) or ala prima, in one session. Others paintings are larger 18"x24" to 36"x36", completed over time in my studio.
Before commencing a painting, I do several 2 minute 2"x3" tonal thumbnail sketches with a marker or Biro pen.
I often prepare colour notes before commencing a painting. Mixing colours (tone, hue and intensity) for the three main compositional areas in the painting composition, by comparing the paint mixes (colour notes) with the actual subject, using a "colour and tone isolator". A small card board with five gray tones plus black and white that have a 5mm hole punched in the centre of each tonal area to aid my analysis of the subject.
While speed is not the aim with my plein air painting, limiting the painting time seems to enhance creativity, probably, because it leaves less time for my logical brain to take command and enhances my ability to paint in a loose and painterly manner.