Caulk around window sills
It's important to reapply the caulking / silicone, but not vital that it happens immediately. You'd probably want to do it in the next few weeks though, as it gets into the summer season. And when you reapply the caulking to this window, you should do it to all of your windows.

Gaps around door and window frames are one of the easiest areas to fix to reduce energy usage in your home. Given the state of the caulking around the other windows, though, the lack of caulking around this window is probably not going to cause a noticeable increase in your A/C usage - it's been increasing steadily - though slightly - over the years.

Caulk degrades over time, as it becomes brittle and cracks, and needs to be replaced every so often. Your A/C has been working harder than it has in the past because of this. This is why it's a good idea to replace the caulking once it starts to crack.

Silicone is (supposedly) more resistant to cracking than caulk, but may still need to be reapplied at some point.