Disclaimer: I have no connection with this service. I am not an affiliate, or an investor, nor am I acquainted in any way with the company or any of its officers or employees.
From their website:
The Free plan allows you to monitor up to 50 sites at 5 minute intervals.
The Pro plan is $4.50 USD/month (if paid annually; else $5.50/month) and allows you to monitor more websites at 1- or 5-minute intervals.