HW crypto isnt going to change battery life much at all. I have multiday standby life on n900 with openvpn running. The big consumer is probably the radios, it's important to configure openvpn on both server and client to not do excessive keep-alives. Remember, on 3G, one single packet every 5s is in worst case same as a voice call, or even worse. Back it off to 20s and worst case becomes roughly 16h standby, so 60s is probably minimum.