The time it takes to mow a lawn depends on the size of the lawn.
I use a riding mower and mow my small lawn within 20 minutes but when I used a push mower it took me around 35 minutes with taking breaks to cool off and rest.
If you have a larger lawn and only have a push mower then it can take an hour or 2 hours to mow the lawn completely.
You can stop and take breaks every 10 to 15 minutes which will make the lawn mowing with a push mower much easier although it will take a little longer to complete the mowing of the lawn.
With a riding mower I can usually mow an acre of lawn within an hour or 2 hours depending on if I stop and take a break here and there.