Note that different recommended shot time intervals exist for the different cueing sports and even from league to league in the same discipline. Below are some references that you may find of interest:

Amarillo Lone Star 8-Ball League Slow Play_Shot Clock Rule

This site has a thorough and thoughtful description on the issue of players’ shot times and slow play. I particularly liked their outlined first step. It begins, “There is normally no time limit for you to take a shot.” Then the paragraph continues, “However, a player may implement a shot clock if they judge that you are delaying a match unnecessarily…” Their discussion is, I feel, worth reading.

American CueSports Alliance Rules

The American Cuesports Alliance refers to the World Standardized Rules, in particular, locate their Rule # 6.15 “Slow Play” and Rule 6.16 “Unsportsmanlike Conduct”.

The Arizona Women’s Billiard Tour Rules

The Arizona Women’s Billiard Tour addresses “Slow Play” in their website’s right sidebar, so scroll down to that heading (beneath “General Ruls and Guidelines) to read their information.

Rules for Russian Pyramid in Wikipedia

These are the rules for Russian Pyramid as listed in Wikipedia, see Rule #29 “Slow Play”

The New Illustrated Encyclopedia of Billiards:  Completely Revised and Updated

“The New Illustrated Encyclopedia of Billiards: Completely Revised and Updated” by Michael Ian Shamos.

This links to and the description of Mr. Shamos’ excellent book. In it, he includes a number of relevant entries about shot times. See p. 79 Delay, p. 95 Extension, p. 112 Game Clock, p. 221 Shot Clock, p. 227 Slow Play, p. 247 Suspension, p. 258 Time Limit, p. 258 Time-out, p. 259 Time wasting, and p. 277 World Team Billiards.

Official Rules of the BCA Pool League 2009-20010

Official Rules of BCA Pool League 2009-2010. See under General Rules, Rule 1.16 “Shot Clock Procedures” on page 26.

Mosconi Cup Shot Clock Rules

Mosconi Cup Shot Clock Rules, explained on first page, under ‘Format’.

USA Pool League, Match Play Rules

USA Pool League, Match Play Rules, see #24 “Slow Play” on page 11. This league follows BCA rule 1.16.

The APA Official Team Manual, see Page 3

The APA Official Team Manual, Page 3. See the tables of “Time Guidelines”. Note that these are listed as ‘guidelines’ rather than as ‘rules’. The 20 second guideline for a normal shot is the lowest value I found for normal play, professional or amateur.

Premier League Snooker Rules

Premier League Snooker, go to “Rules” to read entry under “Timing”. In addition to the 25 seconds that a player has to complete each shot, these rules also specify that 5 time-out’s are allowed per match with no time limit.

The Association for P.O.O.L. Rules

The Association for P.O.O.L., see page 15 under “Slow Play”, “Shot Clock”, & “Time-Outs”.

Valley National 8-Ball League Association Rules

Valley National 8-Ball League Association, see under “E. PLAY”, #6 “Slow Play Rule”.

World Pool-Billard Association Rules

World Pool-Billard Association, see Rule 19 “Shot Clock”. It is interesting that, when using a shot clock, these rules allow 35 seconds per shot but only 25 seconds for an extension. More usual is an extension equal to the allowed shot time.