What is the best way to create a Purchase Order that will remain open for up to 12 months for multiple services/invoices?

I can show you how to create purchase orders and how to manage it, @eandiholdings.

After creating the purchase order, you can keep that one Open as long as you won't manually close it or add it to expenses or bills. You can generate a purchase order from the + New button. 

Here's how:

1. Go to the + New button and choose Purchase Order. 
2. Pick the vendor from the Vendor ▼ drop-down
3. Add all the necessary information. 
4. Click on Save and close once you're done. 

See this article for detailed guidance: Create and send purchase orders in QuickBooks Online. 

To manually close it, open the purchase order and click the Open drop-down below the Vendor and select Closed. 

The other way is to add the purchase order to an expense, bill, or check. You'll just have to create an expense, check, or bill and select the Vendor name from the Payee drop-down. Add the correct purchase order from there. The purchase order will automatically close after clicking the Save and close. 

See this article for your reference: Add purchase orders to expenses, bills, or checks in QuickBooks Online. 

You can run an Open Purchase Order List report to view all your open purchase orders. Here's how to run a purchase order report in QuickBooks? article for more information.

Drop me a line below if you need further guidance or questions in creating a purchase order. Take care and have a wonderful day!