Health and Welfare Benefits

Our firm is looking at switching paying the H&W benefits in cash to providing health and retirement benefits.  Basically the entire amount of the benefits will either be paid to health insurance and if any left over after paying for the health insurance the rest will be put in a 401k.  However, the health insurance is paid on a monthly basis and the benefits are earned on an hourly basis.  If someone works less than 40 hours, they may not have enough benefits to pay the health insurance. 

My question-does the determination have to be made on a pay period by pay period basis or can you look at several weeks at a time?  How are others doing this. 

Any comments are appreciated.