During a client briefing at its New York offices Thursday afternoon, Nielsen indicated it has 60-day to make a decision on its new time-based system for prompting viewers to record their TV viewing in people-metered households. Attendees said Nielsen executives appeared to be leaning toward a plan to prompt viewers every 21 minutes. Another option is a 42-minute prompting interval. The shorter interval is expected to yield higher ratings and persons usage levels, but some have raised questions about whether there is sufficient research to support that decision, as well as concerns that the shorter prompting intervals might interfere with the process of TV viewing itself. For example, a viewer watching a two-hour movie on television would be interrupted six times to reaffirm their viewing on Nielsen's people meters. -- J.M.