Hey Guys!

I am first time here in Skyrim forums.

Could you be so kind and help me out?

I am here to ask about The Companions quests...

For example I already finished many of the Companions quest called 'Animal Extermination'

Are they infinity quests or one day they will stop give me this quest?

Same as with Dawnguard in Vampire Castle.

Guys are giving me ( as I recall it right ) 2 or 3 different quests, but they are repeatedly, just with other location, but object is the same.

Please tell me. Is it worht trying to finish them all or it is useless?

The radiant quest system means you can continually return and get such quests till you throw your hands in the air, there is seemingly no end to them. After a certain point you might just want to drop them altogether as you're considering.

Edit: If you want a radiant quest line that actually means something join the thieves guild.

BTW welcome to the forums OP.

Yes, once you finish Glory of the Dead, the only thing they give you are Radiant Quests, and if you want those, the missions from the radiant quests from the Dark Brotherhood or Thieves' Guild are much more lucrative.