If it's a steel frame and the dent is small enough that you'd ask, then odds are great that is won't matter for 10s of thousands of miles.
Simple dents in steel frames are very common, with one of the most common causes being a handlebar impact when the front wheel turns in a crash. The only serious consideration is the damage to the owner's pride, and BITD it was common to do whatever was needed to hide the damage, then see a hypnotist so you could forget about it.
Of course I can't say anything for sure without at least seeing a good photo of the damage, but unless this dent is deep, with some buckling of the tube, you've got plenty of time to fret over it.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.