Bottom line:  There is little published evidence evaluating frequency of monitoring level sof sedation.

Joanna Briggs did not have any information.

CINAHL and PubMed had guidelines and validation studies evaluating the various sedation assessment scales, but none of that literature addresses how frequently to perform assessments.

DynaMed references recommendations of American Hospital Formulary Service.

Most relevant results

Brook AD, et al.  Effect of a nursing-implemented sedation protocol on the duration of mechanical ventilation.  Crit Care Med. 1999 Dec;27(12):2609-15.
Reassessment every 4 hours is part of the protocol; outcomes were reduced time on mechanical ventilator, length of stay in ICU, and rate of tracheostomy for the protocol group compared to the standard care group

DynaMed

Propofol drug information.
Recommends assessing level of sedation at least daily.  See Warning and Precautions>General Precautions>Critical Care Sedation

Guidelines and additional validation studies:

Clinical practice guidelines for the sustained use of sedatives and analgesics in the critically ill adult.  Crit Care Med. 2002 Jan;30(1):119-41
-recommends using a validated scale to monitor level of sedation, but cites a systematic review of scales to state that there was no gold-standard scale for assessing sedation level at time of these guidelines.  Does not recommend specific frequency for monitoring patients.  Objective Assessment of Sedation section reviews evidence of Motor Activity Assessment Scale, Riker Sedation-Agitation Scale, and Ramsay Scale and Vancouver Interaction and Calmness Scale.  Does not include the Richmond Agitation-Sedation Scale.

Ely EW, et al. Monitoring sedation status over time in ICU patients: reliability and validity of the Richmond Agitation-Sedation Scale (RASS).  JAMA. 2003 Jun 11;289(22):2983-91.
-Confirmed interrater reliability and validity of RASS in medical ICU patients.

Vender JS.  Sedation, analgesia, and neuromuscular blockade in sepsis: an evidence-based review.  Crit Care Med. 2004 Nov;32(11 Suppl):S554-61.
-Includes discussion of scales in context of patients with sepsis