Neither opinion nor objectivity are particularly tightly defined hence any statement as to the point at which they change state is, by necessity, opinion (somewhat ironically).
However, what I like to use as a yardstick for objectivity is if it is feasible for others to come to the same opinion and, by applying reasonable rules of evidence and logic, they will come to the same opinion.
Using this approach, MarkOxford's "I find tomato's tasty" could never be objective but "tomatoes grow on tomato plants" can be.
You may note that what I've really done is replace the, somewhat vague, terms of opinion and objectivity with the, also vague, terms of feasible and reasonable. I don't believe there's a better approach but that, of course, is just my objective fact.