How long do we keep tenants stuff after they leave?

[Curtis M.]

Purchased a duplex in Colorado and I'd like to know how long we are required to store/hold onto the tenants property they left behind after they move out? 

The tenant has agreed to leave and has signed documents stating they are leaving on Sept 10th. The house is currently unattended and boarded up due to law enforcement. Is there a required timeline or do I need to send notification to the tenant letting them know my intentions if they choose to leave belongings behind?

Any help would be appreciated.

Thanks,

Curtis