Which i7 and which i3? Are we going to compare the 2600k to 2100, 2120, or 2100T? Are we including mobile versions in here? Which programs in particular? Will they make use of the extra cores from the i7 or are they only dual threaded.
Anyone who gives you a solid % answer is making it up. You need to ask a specific question if you want such a specific answer, but in reality your best bet would be to look at the benchmarks yourself to get a better understanding of how their performances in different tasks compare because it is much too complex to sum up in a single percentage.
+1 you need to know which exact CPUs to compare and also with games most will be limited by the GPU anyway and even those that are not some can make better use of the extra cores on an I5 and hyper threading on the i7 and some cannot so its an impossible question. You can compare different CPUs at http://www.anandtech.com/bench/CPU/2