I wouldn't base this type of maintenance (valve stuff) on time, I'd base it on mileage. Now for
oil changing, some will need to base that on time, not mileage. Low miles, "over time" generally means the
oil will be contaminated.
One of the good things about mechanical lifters is, it keeps you in touch with your motor. You have a REASON to be in there looking at stuff and as a consequence just might find a problem before it becomes a problem.