Its like asking how long is a piece of string. Properly maintained, of a suitable wood such as Sitka Spruce or old growth Douglas Fir, I know of masts more than a 100 yrs old. Judging by the general nature of your post, I would think that this is a new area for you. In which case I would suggest the mast be pulled, and an experienced woodworker/mastwright look at it closely.
so many projects--so little time !!