Well, that will not really rebuild your index. It depends on how often your index job is running and how many items will be indexed at each run. If a user has thousands of items it will take several days ....
To run manually an index job look at this: https://ohmag.net/ask-the-experts-groupwise-39/, especially the last lines.
If you want to know if you have an index issue, check your gwcheck logs. At each user chapter there is a final statement telling you "pending job". If there are many pending jobs (>1000) for a user then probably a manual index will help.
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