The timing of patching depends on the level of vulnerability and systems exposure. The general rules is 24-48 hours. No patches go into production without systems testing first.
We perform software feature updates 6 times per year. Merchants will have 3-4 weeks advance notice prior to an update. The system downtime is generally 2-5 minutes. We are currently implementing a high availability update procedure that can reduce downtime to zero.
Hotfixes are implemented as needed and generally require a reboot of each web server. Because we have a server farm there is no effect to merchant processing.