you have to keep in mind that for your setting to work you'll have to set external IPs as trusted IPs for MFA. So for example when you access your resources from within your company's network you don't have to use MFA whereas you will need to authenticate via second factor when accessing from outside of your company. Just as Nils mentioned your settings should be active almost instantly.
Best Response confirmed by
Dean Gross (Respected Contributor)
Either check some webservice (google: my ip address) while you're at work, or talk to the IT/Network team and let them tell you the public IP range (ip+subnetmask bits) if you're using more than one IP address.