We have a
busy 4 man shop.
1 billing person/admin assist/exec secretary
myself and 1 very key person and a 2nd pension administrator.

What is reasonable to expect from that key person in terms of hrs
we have 16 vacation/sick days
person does not work any ot or saturdays.

Is it reasonable to expect some ot and sat work in office this size?
Does answer really lie in compensation?
ie
if hrs worked over the course of the year are substantially more than the normal 37.5 for 50 weeks should it be reflected in a christmas bonus?

If both the employer and the employee are in agreement then the expectations would seem to be reasonable.

If you are asking what is reasonable from the employer perspective because the employee doesn't want to work as long as expected, the question probably does come down to a matter of compensation and how key this person really is.

I'm not sure hours worked is a good measure anyway. IMHO, their contribution toward the company's bottom line is the most critical metric. I've seen several firms where hours worked was a big deal to them, but the most efficient and most productive employees would rarely receive recognition/rewards, even though they added as much or more to the bottom line while spending less time to get there.

If you need them to take on additional work, which could require additional hours, I still suggest that any additional compensation related to such extra work be based on the results, not how long it takes to get it done. I have also seen many procedures that were unnecessarily lengthy to support manual work because the people doing the work were unable to use excel or VB to speed things along. Be sure to measure quality as well as quantity.

Thanks and that's a good point.
We have over last 2 years given employee raises of 22 and 25%based on abilities and level of work.
What I am starting to feel is for those type of increases based on quality of work;giving more than 49weeks of 37.5 hrs wasnt asking too much.