I do 30 seconds walk then 60 seconds sprint for intervals based upon 15 -20 sets. When I say sprint I mean flat out as fast as you can go! If this becomes too easy (it hasn't yet) then I will drop the rest to just 15 and keep the sprint to 60 seconds.
I would suggest after doing after your weight lifting session and using the HIIT routine that is for 10 minutes 20 sec on 10 sec off each excersise for four sets such as Burpees for 20 sec 10 sec off x 4 continuous sets followed by light kettle weight swings 20 sec on 10 seconds off 4 sets followed by jumping jacks followed by body weight squats followed by straight planks try not to do this the days you do legs because the legs take alot out of you.