It doesn't take much time to learn once you have the lights to play with.
Everyone starts out making films with household-powered lamps (2K and under) so over time, you pretty much learn the limits and range of that approach. So it's not hard to imagine how much brighter a 5K or 10K would look instead of a 2K. If you're always putting a 2K through a 4'x4' frame of Light Grid, for example, and always getting an f/4 when the frame is about six feet from the actor, using 500 ASA film let's say (I'm just making these numbers up!) and you know for this bigger movie with a larger set where the light has to be twice as far away but get the same stop, or conversely, the same distance away but you want to stop down more, you'll probably try a 5K instead of a 2K.
Within a day or two on a film set, you get used to what the lights give you in different configurations, for the speed of the film you are using and the stop you tend to want to shoot at, so your guesses as to which lighting unit to call for are pretty accurate -- and when in doubt, you tend to ask for a brighter lamp, figuring you could scrim it down if necessary.
It gets harder when you start talking about lighting HUGE sets or spaces -- often in those cases, you get the brightest lights the production will pay for. But there is photometric data on these lights so you can make a good guess if you are shining the light directly.
I bet that if you had a 650 watt Tweenie as your only movie light for the next few short films you make, you'll quickly learn just how much light it puts out in different configurations, and you'll be thinking "I wish I had a 1K for this shot because a tweenie just isn't quite bright enough."
