I divide up computer program running times into the following broad categories:
Instantaneous
Terminate seemingly instantaneously after launch, often taking less than 1/3 second
Can get results immediately—this quick feedback is essential for rapid iteration and refinement of algorithms
Extremely amenable for use as sub-routines in larger programs because they can be run hundreds of times within minutes
1 to 30 seconds
Still really fast but not instantaneous—there is a noticeable but not overwhelming delay
You can still get results immediately so that your attention doesn't get distracted by other activities
You can still iterate and refine algorithm many times with rapid feedback
However, not as amenable for use as sub-routines in larger programs because they can only be run a few times within minutes
Low number of minutes
Usually terminate in 1 to 10 minutes
You can answer emails, surf the web, or run other errands while waiting but at the risk of losing mental context and concentration
A bit annoying because you don't want to wait around for it to finish but can't really do much else useful in the interim
High minutes
10 to 45 minutes
You can squeeze in a few runs during the course of a work session, but must interleave with other activities
Implementation bugs start to get annoying because it's starting to take a long time to run the program
In order to refine and tweak parameters, you can automatically run it overnight in up to a dozen different configurations
Hours
45 minutes to 4 hours
You can only run it a few times per day
Fortunately, it's easy to do other work in the interim because there is enough time to complete other tasks while this one is running
Overnight
4 to 10 hours
You can run once per day—usually run it overnight and analyze the results the next morning
Implementation bugs get especially annoying because an entire day's worth of work is wasted if the overnight run fails mid-way through
Because it's running overnight, it's often difficult and inconvenient to monitor the run
Usually a good idea to debug program by testing on smaller inputs that only take a few minutes
Days
1 to 7 days
You can only run once per work week
Must be extremely careful about implementation details because the consequences of a crash or incorrect behavior are more drastic
Must think about building in monitoring and self-repair facilities into the software so that it is less likely to crash mid-way
Maybe prefer fail-soft versus fail-fast software design—i.e., prefer getting incomplete results than crashing with no results
Almost impossible to iterate rapidly because the turnaround times are so long
Weeks
1 to 4 weeks
Definitely want to implement fail-soft and constant monitoring and online adjustment facilities in the program
Must be able to 'steer' the program's execution while it's running in order to account for unintended conditions
Must be able to obtain incomplete and partial results incrementally
Forever
Anything that takes longer than a month to run
The results had better be worth the complexity of the algorithm!
Good luck ;)
