How to determine the amount of tips to give?

Usually, if the mm provide extra service or slightly overtime, I will give tips and won't make her work for free.  If the service is standard and I leave on time, I usually don't give tips.  For the repeat often mm, I decide to book longer session, such as 45 min, if I don't give tips in this case, will the mm be disappointed?  For half hour session, I usually give $10, or $20 occasionally, but for 45 min, will they expect the tips to scale up according to the amount of time, like expecting $20?  If I have always giving then 150 for hh, but when I switch to 45 and no tips, will they provide lesser service or not as great attitude on the next visit? Fellow wolf bros please share your opinion on this.