From how it sounds now people might think all the stacks are removed after 25 seconds left since last creep.

I recommend rewording it to: "Alchemist gets bonus gold for killing units, for each consecutive kill gaining bonus gold. Each stack has independent duration after which stack is removed The bonus gold is capped at 30 gold." (not ideal wording)

Plus add a buff indicator that will show how much gold you will get for next killed unit.