Every user and/or experts is different and will take a different time to answer. You can have an indication of what you should expect to wait by going to their profile and looking at their response time. This is just an average indicator. Times might vary greatly depending on the day, time zone and many other unforeseen circumstances.
You can also decide to visit the chat conversation, click on the $ on the top right and submit a rating. You can always change that rating at a later stage.
To delete a message or a question go to the question or chat tab and simply swipe left on the message you want to delete.
More often than not, a user that has a lot of good ratings and an acceptable response time will answer. It is just a question of waiting for them to do so but you always have the option to delete your message and proceed with another user.