Here's sort of a guide.....

Long story short (if you want a longer version, read up on zone system), your negative gets twice as dark every time you extend the development time by 20 to 30% and contrast goes up by one grade. That means if you are printing, your exposure time under the enlarger gets twice as long every time you develop your film 20 to 30% longer. This is not exactly correct but it'll give you sort of a visual on what happens.

You can certainly successfully print these images although that long of an exposure time may become painfully boring after doing it so many times.

This works by percentage by most part. So longer your dev time, loser accuracy you can afford in terms of absolute seconds.

Seriously though, don't stress so much about timing issue. Get it reasonably accurate, say 15 seconds (in my case) and you'll really not notice any difference.