In addition to openings, is there any ventilation method?
No. If you are worried, you can use a drill to drill a few small ventilation holes to allow hot air to rise from the top of the side and exhaust. This will keep bats cool and safe, away from elements and predators.
How long will it take for bats to move in?
As bats are wild animals, it may take several months for local bats to discover your bat house, So have patience, they are fascinating creatures.