There are a number of factors you need to consider:
How many apps are being supported ? - this can impact schedule of when to run refresh of data, processing power requirements etc
How many users ? - this will impact licences
What technology/systems is QlikView extracting data from ? Does it have any downtime/housekeeping windows ?
Sometimes a company may have an SLA (service level agreement) which defines how long a service can be unavailable, when upgrades can take place, who you should escalate issues to and the classification of severity of issues etc.
