Terms for Client Agreements

so I’m relatively new to the website design business. Been doing it a little over two years. About 1 year now with WordPress, pretty much exclusively with WordPress.

I’m running into more instances now where my contract is expiring or has expired. But their themes or plugins need updating.

When I write-up an agreement for a NEW client whose site I’m going to build, what sort of terms related to theme/plugin updating should I include? I don’t want turn off a client by saying “I’ll be building the site with wordpress. And I’ll be doing all updating to the site for the next 12 months. But after that you’re on your own for the updates if you don’t want to continue with me.”

With HTML sites I was building with dreamweaver, this was not so much an issue because there weren’t constant updates to themes and I had no plugins. So there was no reason for terms like this. But now I’m not sure how to address this.

Any thoughts? please advise. thanks.