Most natural veg dies within a few hours. However, this is not necessarily a bad thing, depending on where you make your hide. For example, green veg turns to brown when it dies, which works very well in field areas, with long dry grass. Some of that veg is stuck in the ghillie for months, and has never been removed. Other veg falls out, or dies off and deteriorates.
To add vegetation, use dental floss to tie it on tight. It will stay there, and will not fall out. Works much better than putting it simply through the netting, as its more prone to falling out.