increasing intensity over time

GaiaGirl1992

I currently do intervals on the treadmill/elliptical at the gym, I started out doing 1 minute as hard as I could, then 1 minute rest, etc. and add a minute every week or two to the intense part, and leave the rest time at about a minute. currently, i can do 2-3 minutes at higher speed and resistance for around a half hour total (10 fast-rest rounds).

what I'm wondering is how often is it okay to increase the intensity of a workout? is 2 weeks enough time to gain the stamina and strength, and still be challenged?