We do not have formal guarantees for uptime for shared hosting plans. However, in practice, our monitoring system shows our shared webhosts are up and responding correctly 99.99% of the time. If upgrades are required for shared hosting servers requiring more than a minute or two of downtime, we schedule these in our webhosting maintenance window from 1am to 3am each day. In practice, these upgrades do not tend to take longer than an hour or two and are not normally required more than once a year.
If you require better uptime than this, or you need to have these necessary outages arranged with you, we recommend one of our colocation or premium hosting plans.