> "... Then you should decide on good,regular times for studying."
Time is allocated in separate chunks at regular intervals and in ways
that are appropriate for the circumstances.
> "...Then you should decide on good,regular time for studying?"
The period of time itself must be well-chosen and "regular," whatever
that might mean (always under the same circumstances, perhaps?).
> Then you should decide on good,regular time for study."
Essentially the same as the second sentence above, but "study"
emphasizes the concept a tiny bit more, whereas "studying" emphasizes
the action itself.

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