When am I supposed to get an acknowledgement for a paper submitted via e-mail (CS conference)?
In computer science, paper submission by e-mail is non-standard. Therefore, there is no default answer to your question.

However, it's in the interest of the conference organizers to show good professional communication that does not leave authors "in the dark". I would expect them to confirm your submission immediately after submission, within 24 hours. If you have not heard back from them after 24 hours, it's absolutely OK to contact them.