The Greek striker has scored 63 goals in 218 appearances for Celtic since joining from Manchester City in 2008, initially on loan.
He has a year remaining on his current contract and, ahead of Wednesday's UEFA Champions League qualifier against Cliftonville, he continues to enjoy his time with Celtic.
"I enjoy my football here," said the 28-year-old. "I'm at a club where I feel well.
"It was important to me to find an environment where I feel good because I'm away from my country, my family and friends.
"I need an environment that suits me and here I am. I'm not surprised how long I've been here. I always felt I'd be here a long time. I feel great at this club and that means everything to a footballer.
"Celtic made a lot of sacrifices for me and I did for them also. We have an understanding relationship and that's why I have been here for nearly six years.
"I have a great relationship with the manager and, if he wants me, I want him as well.
"I'm still young at 28 and I can see myself here for a few more years. I'd never say no to a new contract, if they offer me something we can discuss it.
"Will I finish my career here? It's difficult to say as I don't know when I will finish. It could be 33, 35 or 37."