I think its probably just up to the GM. The only time you will most likely need to make a call on the exact time is if you bump up to weeks or months depending what might be going on in your campaign or setting.
Agreed. I think this rule is designed to keep you from stopping at the corner store in the middle of a bank heist to pick up a TAG and a few Missile Launchers last minute. Can the heist wait a week? Yes, ok, then I'll spend some time trying to find that TAG. No? Ok, then we go in with what we've got!
"Each step along the following
track reduces the difficulty
of the acquisition test by one
step, to a minimum of Simple
Weeks—Months. If the difficulty
is already Simple (D0), or has
already been reduced to Simple
(D0), then each additional step
adds 1 bonus Momentum to
the acquisition test."
So it's important to note that usually things arrive within minutes, waiting more than a day for something gives you -2 to Availability.
Yeah like darkdaysdawn and janorichard said, just a GM call on the actual time. I guess you could set up a system to roll for it. My plan was generally to have cuts between different scenes like 'a few hours later' or '6 months later' etc. during which PCs can get an automatic reduction for acquisition tests by assuming they went shopping or made some calls to shady missile launcher dealers. Maybe they put down a deposit on said missile launcher and it was just delivered via spaceamazon.
It's always going to be up to the GM exactly how long it takes.
If you go by base, it could be as little as 60 seconds or as many as 59 minutes, up to as many as 11 months (if it's 12 months or more I'd say that's a year, so a different increment) if you want to step down the difficulty by -4.