There are usually 2 maintenance AWS does.
- Continuous managed maintenance updates.
- Service updates
While creating cluster you need to specify a 60 min maintenance window. Usually all the maintenance updates (1) will happen during that time.
For every service updates you will recieve notifications when there is a scheduled one. Notification will be in the form of email or a notification on the elasticache page etc... Based on the notification you can reschedule the service updates to a comfortable time. If you fail to reschedule it will by default pick you maintenance window and apply the service updates.
Basically during the maintenance updates, AWS will replace your node with a new node with required updates. If you have primary/replica set up with multi az and auto failover set to true, then during maintenance window of the primary node, you replica will be promoted to master and your read/write requests will be served from there. So ideally you don't see any issue during maintenance maybe a few second downtime to promote the replica as master.
If you either don't set up multiaz with auto failover to true or your elasticache has just one node, you will see downtime during maintenance window.
Refer AWS documentation