say the dancer is just sit your lap, chatting for you and no dance. then how much for tip her "for her time" per hour or per song?
it must be less than cost of true lap dance, yes, since sitting is easy?
or just buy girl an alcohol is enough tip if she sit in your lap for 20 minute?
sometime a girl is interesting personality, but I decide I do not want dance from her, yet wish to time for her time


You don't tip her anything for her time. This has been discussed here before. If anything in this situation ask her when she is on stage and tell her you will come see her then. Never pay a dancer for just sitting or speaking to you, you are a PL not in human.