And it's not
finished until it's good, right? Haha!
But really, as for time, depends on your workflow - if you're using all hardware and doing instruments yourself (i.e. live) think about it - If I want the beat to loop every, say, 8-12 seconds, then I should lay drums for 1 or 2 minutes, grab whats good, get the computer to quantize it or whatever, then lay out a bass line, play around, basically jam with myself, then guitar, uke, bongos, breath noises, body beats/slaps, beat box, whatever, it's basically sampling yourself so no digging involved, then you can get an arrangement in 15-20 minutes EASY. but MIXING could take hours.
If you're using pre-loaded kits in Reason/Ableton, and synths or pre-loaded samples, then again, it's just jamming with yourself on loop, so it's easy to bang out a bitching
LOOP in 15-30 minutes (maybe 30-45 if you want an intro, hook, break, outro).
If you dig, rip, tweak, and layer [/i]each sample[/i] then I could see it taking days, but most people don't do that unless they're composing a master piece (or trying to, or making something specific for an artist, etc)
Will all your beats be good? Hell no - did you ever sit down at a keyboard (or whatever instrument is easiest for you to goof off on) and play some melody or progression that was HEAT? sure! did you ever (in the same 30 seconds of playing) get 15 seconds of notes that were corny as hell? Definitely. Sometimes we don't realize that the progression or rhythm or melody is whack until it's too late.
But don't look to strangers for approval, look within yourself, and if you think your material is good, then good on you.