This question is ambiguous and confusing to me. More context is needed. "bucket span" may refer to indexed data buckets on disk, or it may refer to the span of time for search results aggregations produced by, e.g., timechart
or stats
. Please make this question more specific. It would also be useful for posterity to explain or link to some explanation of why such a thing might be useful to know.
use dbinspect to see the bucket span
http://www.splunk.com/base/Documentation/4.1.3/SearchReference/Dbinspect