This depends on many factors, so it is quite unanswerable.

• Priority: It can be few days for High priority bug fixes to several months for Low priority bug fixes.
• Reviews: All fixes needs a review, so sometimes it taks several days if it needs to be rewritten.
• Version: Some version aren't supported anymore. For old versions bug fix backports are only done with OpenERP Enterprise. For last stable, it really depends on the priority.
• OpenERP decision: Depending on the reported bug, it can be defined as an opinion or a wished feature.

An option to accelerate the process is to take a support contract (OpenERP Enterprise).

Here is a link of stats on merge proposal provided by Fabien in response to How active is the OpenERP Community:

• OpenERP Community Dashboard

A part of the community think fixes are merged too slowly or the opinion diverge on whether or not it should be fixed. So some partners make there own replicate of the official OpenERP branches.

Lastly emerged for the v7 some other replication which also intend to merge faster the bug fixes.

Those branches are maintained by some of the community members

Those are the OpenERP community Backports:

• OpenERP Community Backports (Addons)
• OpenERP Community Backports (Server)
• OpenERP Community Backports (Web)