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All depend on your scenario. No hard and fast rules here.
To be on the safer side compress the least active tables first. For larger tables, choose the least active partitions.
On average you can reduce space usage on tables by 60%. But it totally depends on the profile of your data.
Time it takes to compress depend on the amount of data you have ( rather than the no. of tables ) and the power of your server.
You can take the compression in low-active windows. Don't take the all the tables at once. Segment the tables and take one segment at a time.
It's not just compressing the tables. Existing data is compressed by rebuilding table. When a table is rebuilt indexes also should be rebuilt.
May be it's worthwhile to consider compressing indexes also. In that case, you may need to drop the indexes and re-create them compression enabled.
Trace files, core dumps, listener logs, archive logs