Another on the bandwagon for, "If you're playing within a structure over changes, everyone should know when to come in because they've been counting along, regardless of who's soloing."
If it's the traditional jazz 32 bar form, everyone gets 32 bars. Everyone should expect every solo to end after that player's 32 bar section.
If it's a song where the solos are over a specific section of song, say, "the chorus", everyone should know how many times around the chorus each solo is, and then be counting and ready to come in after the appropriate number of section repeats.
If it's free-form jazz, then it doesn't matter and everyone should just play whenever. (i'm mostly joking with this one, but also, a tiny bit serious.)
Musically, it's nice to have your solo either have a nice distinct conclusion, or blend seamlessly back into the next section of song. One of those styles will be more obviously ending than the other, though.
A solo is your time to speak, and what you're saying musically is hopefully more than just playing a bunch of arbitrary notes in the key. If it's not obviously ending or blending right back in to the head, what are you saying with your solo? When you're orating, you don't end a speech by saying "And now i'm done with my speech." It's how you say your conclusion that tells the listener you're done. Same goes for a music solo. If your bandmates are waiting for you to end your solo with "and now i'm done with my speech" (but in musical note form), that would be silly. They need to trust you'll end your solo in your allotted time, and you need to trust they're keeping track of time. And also, if your solo doesn't lead back to the head (or next section or whatever), is it really that musical of a solo?
And if everyone in the band is just soloing over arbitrary amounts of time regardless of the structure, form, or changes, maybe have a discussion about that. Because part of what gave jazz players the freedom to play was the fairly strict and predictable structure to play within.