well... that will take a long time...
should be right: אני עדיין מחכה לחמישים
Without שנות (years) , would this imply the fifties isn't talking about a decade but something else.
aní adáyin mechaké li-shnót ha-chamishím.
(colloquially, it will be "le-shnót")
Why can't עדיין come first?
It could, but it would be require a specific emphasis, stressing the עדיין. Not sure why you'd want to change the word order, though, as this word order follows the English word order and makes it much easier when translating.
מה ההבדל בין ממתין ומחכה?