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I flew to Rhodes last year with Monarch and the flight, although it was meant to be 4 hours, only took 3hrs 20mins. Not sure if that helps. It actually doesn't feel that long in the air - no sooner are you up there, the crew are coming round with magazines, drinks, earphones etc....and then it's dinner being served (if you've opted for that).....then one last push on the sales trolley and you're landing. Rhodes is lovely and that's your reward on the end....just focus on that and the things you would like to do when you get there.
Planes do bounce slightly, remember they are designed to do that. But it's really no more servere than the bounce you get on the train....the carridge rocks back and forth, side to side, click clacks etc - it's much worse sometimes but do we bat an eyelid?
The recent turbulence event in the news is very rare and occurred over the Atlantic, where a lot of storms develop - anyhow, they all made it back. You're flying to Greece, where the air is often dry and sunny. You'll be fine, I just feel it will be ok for you.
Have a great time