I have 2 Jack Russell Terriers at home - one is 1 year 9 months old and the other is 1 year 3 months. We adore the Jack Russell breed, just adore them.
However, when we got the first puppy we had NO idea what we were getting ourselves into....we were stupid and didn't do any breed research what so ever. We kept the puppy, did a lot of research and took training classes with the puppy and made it work, because we feel in love with the boy. We ended up getting a second one because we fell in love with him too. We LOVE having 2 - they play together and wear eachother out. It took a few days for them to get used to each other, but now they love eachother like they were brothers.
Jack Russells are VERY energetic and VERY smart which can lead to problems. They need to be exercised and played with and trained. If left up to their own devices they will tear your house apart! I'm lucky enough to work from home, so they stay out of the kennels all day but still need exercise. We found an off leash dog park that we go to several times a week, we play fetch in the house, work on tricks, and go for walks.
I think as long as you have them crated during the day while you are gone with lots of things to do (aka. Kongs, buster balls) they will be fine as long as you make a commitment to exercise their mind and body.
I'm not trying to discourage you from getting a Jack, afterall I do adore them myself!
They can be such a great pet! Training them from the beginning is a must and they will excel in tricks too! They are a great breed because not only are they small but since the breeding line has been kept so pure there are relativley no health problems (there are no major breed standards, they are breed to work) and they do live for a long time (15 years).
I am a huge fan of Jack's and LOVE our two and all the others that I've met....I'm sure a Jack would be a great addition to your home!
And, just for fun, here are my boys, Sparky & Skeeter!