Narfeldt,
This topic should bring some interesting comments.
From the Automotive Industry Action Group publication on Statistical Process Control page 31. "Subgroups should be collected often enough, and at appropriate times, that they can reflect the potential opportunities for change...Subgroup frequencies for ongoing production monitoring could be twice per shift, hourly, or some other feasible rate." Interesting, but not very useful.
From Statistical Quality Control by Grant and Leavenworth, page 42. "No general rules may be laid down for frequency of subgroups. Each case must be decided on its own merits, considering both the cost of taking and analyzing measurements, and the benefits to be derived from the action based on the control charts." Better, but still not much help.
In Dale Besterfield's book Quality Control he suggests that it is a cost-benefit analysis but if you absolutely need a rule of thumb use the Normal Inspection, Level IV sample size table from the old US military standard MIL-STD-414. As an example, if you produce 4000 pieces a day you should look at 60 pieces. With a subgroup size of four that would be 15 subgroups or about one every 30 minutes across an eight hour shift. If the subgroup size is 5 you would have 12 samples or one every 40 minutes.
Regards,
Rick