Update api-management-howto-integrate-internal-vnet-appgateway.md

Add an expectation of creation duration for application gateway

Author: simonkurtz-MSFT

articles/api-management/api-management-howto-integrate-internal-vnet-appgateway.md

@@ -496,7 +496,7 @@ All configuration items must be set up before you create the application gateway
 
 ## Create an application gateway
 
-1. Create an application gateway with all the configuration objects from the preceding steps.
+1. Create an application gateway with all the configuration objects from the preceding steps. It can take 15 minutes to create an instance.
 
     ```powershell    
     $appgw = New-AzApplicationGateway `