There are two mods here,
free_shipping_weight_limit (only for free shipping weight limit) : is from
http://www.opencart.com/index.php?route=extension/extension/info&extension_id=10242
I just made this into a vqmod, so that you don't need to replace any core files.

free_shipping_alert_with_weight_limit (include alert and weight limit) is a combination of the above and the other free mod,
http://www.opencart.com/index.php?route=extension/extension/info&extension_id=11265
I made several little changes, so that it can work well with weight limit. I also changed the position where it will appear.

By default, i set 1kg limit for free shipping. If you set different value in free shipping module, you need to change the text in xml accordingly.

Only tested on 1.5.5.1
No support is guranteed. I'm simply a user, making little changes to free mod to make it work better.