Invertase as an enzyme will continue to work until forced to stop. One way is heat as mentioned above, another is pH, but not a problem with fondant unless you've added acid for a fruit flavor. The final way it's stopped is if there is not enough moisture in the fondant.
Invertase will work until the moisture level of the syrup phase falls below 20%. Most fondants are about 50:50 crystal to syrup (but can range to 60:40, and about 12% total moisture. So if 50% syrup phase, it's about 24% moisture. So you should see significant softening.If less moisture, just a couple percent, will result in minimal or no noticable effect. However, it all depends on all of the above.... age of the invertase, temperature seen, pH, and then moisture level of the fondant and resulting syrup phase. The level of invertase will effect speed to the end result, not how far. A lot more involved but the above should hopefully help.