My T would just say, "It's time to stop now" but usually would do it when I was flagging :-) or had finished a sentence or thought. I would then get my check out and give it to her and we'd both get up and go to the door so there was still a minute or two for a comment or two or a question, etc. Sometimes she would say it was time to stop and then suggest something for me to think about before next session or otherwise transition that way.