The support pages do not say how often Outlook.com checks your Gmail.com account for new emails but you can easily find a clue in your Gmail mailbox itself.
How frequently Outlook connects to Gmail?
Open Gmail and click the link at the bottom that says “Account Activity” – look for entries that read POP3 with location set as United States and IP addresses in the 18.104.22.168 – 22.214.171.124 range. The time difference between any two consecutive connection attempts in the activity log is the rate at which the Mail Fetcher program of Outlook is connecting to your Gmail account.
In my case, this gap in fetching emails is always 30 minutes.
Bonus Tip: You cannot force Outlook to check your Gmail account on-demand but there’s a workaround. Go to your Outlook Mail Settings -> Send Receive from other Email Accounts, Edit the connected Gmail account and save without making any changes. Outlook will then attempt to connect your Gmail account again for new messages.
Alternatively, instead of using POP3, you can setup auto-forward in Gmail and in that case, the Gmail message will instantly get delivered to your Outlook mailbox.
Also see: Use Outlook with your own Domain