Pick 3 or 4 chords, base melody on chords, make 808s bang on the roots of the chords, ad snare on the 3, hihats every other note with some rolls, kick to accentuate the 808 hit and maybe add a lil syncopation, add another perc like a bell or chime or woodblock. Then to arrange just lead in with chords and melody, drop the beat, do 8 bars with everything playing, 16 bar verse drop out the 808 and hihats, bring hats back for 2nd half of the verse, chorus again for 8 bars, 4 or 8 bar bridge just melody chords and snare, then verse, chorus and finish. 10 minutes if you have good drum kits and you know a handful of good presets so you dont have to hunt for sounds. Then if you wanna export the melody to wav and reverse it and half time it you have the industry standard intro in another 1 to 2 minutes. This is what 90% of youtube producers do almost exactly. Its so formulaic but it can be fun and if ur just making some quick tracks to rap over its fine.
I would say the average amount of time i work on a beat that is good enough for me to want to rap on and release is around an hour. If i really want to perfect everything maybe 3 hours. I would include a sample 10 minute and 60 minute beat but im on mu phone...