Are there guidelines for how many ratings are completed by each respondent participating in the study?

In our experience, up to 20 ratings or choice tasks would be reasonable before the respondents become tired, or bored with the tasks.

One reason is that conjoint evaluations require respondents to consider complex product features, or complex tradeoffs even if the features are simple.

If the design requires more than 20 tasks, you could consider the following options:
(1) distribute the tasks over two or more time periods for the same respondent (however, some respondents may drop off the study),
(2) provide monetary or other incentives to keep the respondent engaged on the task,
(3) distribute the tasks among several respondents who are “look-alikes” using methods such as Hybrid Conjoint Analysis.