This article covers a recent study into online betting limits and is well worth a read.
This bit jumped out at me:
Participants rated positive messages on controlling gambling money as the most helpful and simple to understand. The study determined that deposit restrictions were the most popular among the types of limits available. Overall spending limits, single-wager limits, and loss limits followed.
The least favorite type was a timer. Only 22% indicated that they would use a clock to limit the amount of time spent gambling online.