By default Office365 limits Resource booking days to just 180 days. The maximum days it can be booked for 1080 days.
I like to make resource booking days 1 year from the day of making the reservation/ appointment. Now instead of visiting each calendar and making the change, powershell can help us out.
#Connect to Office365
$cred = Get-Credential
Import-Module MSOnline
Connect-MsolService -Credential $cred
$s = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell -Credential $cred -Authentication Basic -AllowRedirection
$importresults = Import-PSSession $s
#Get all the Rooms and make the booking days to 365
Get-MailBox | Where {$_.ResourceType -eq "Room"} | Set-CalendarProcessing -BookingWindowInDays 365
Happy Booking!