It isn't so much time as thermal loading; picture it as the area under a time/temperature curve.
With an all-grain wort it can make a real difference. There you want a short, low-intensity boil with less than 10% total evaporation (a good indicator,) just enough circulation to achieve a good break and hop utilization. More thermal stress will lead to effects such as the excessive breakdown of proteins and other substances resulting in a much duller, less fresh malt flavor, the development of off flavors, reduced capacity for producing and holding foam, and more rapid staling of the beer.
But Denny's absolutely right at least as regards an extract wort. Most of that damage has potentially already been done in the production of the extract, so you won't make a lot of difference either way. That said, extract doesn't need any more boiling than your hop schedule requires because there's not much of a break to achieve, so no need to use extra fuel; do whatever is most efficient if that's a concern, or whatever is most convenient .