Can the lights be on too long?

Just started some seeds, today they poked through. I am just wondering can I leave the light on 24hrs for a day or 2 or is that bad?

I always leave my lights on for young plants, 24 hours a day until flowering. Wouldn't do that to seedlings if you're using an HPS light though, should use fluoros for young ones.

Many growers keep the light on 24hrs during the veg state so yes, you can.

HPS is good to use, as long as it is far enough from the sprouts not to burn them and if you have proper ventilation.