The first place to look is your state's special education statutes or regulations which may establish specific timelines for school systems to complete evaluations and to develop an IEP. In Massachusetts, for example, the school system must complete evaluations, convene a TEAM meeting, and develop and present an IEP, or an explanation of why there has been a finding of no special education needs within 45 school working days after receipt of the parents' consent. Within that 45 day period, the school system must complete evaluations by 30 school working days after receipt of parents' consent. Your state's requirements might set outside limits between the parents' consent and the school's presentation of an IEP, or they may set time limits for each step of the process, or they may not set any time limits at all.
If your state does not provide any specific timelines for these tasks, you will have to turn to IDEA for guidance on this question. Unfortunately, however, you will find little direction there. Neither the statute nor the current IDEA regulations establish deadlines for identification and evaluation of children with special education needs. (There is an exception under the regulations for special education that pertain to Native Americans, where it is required that evaluations be administered within "30 days from the date of written parental consent" [25 CFR 45.26(a)]. These regulations do require that once an evaluation is completed the school conduct a meeting to develop an IEP within 30 days [34 CFR 300.343(c)].
The U.S. Office of Special Education is currently proposing a revised set of regulations to implement the newly amended IDEA. These include no specific timelines for evaluations either, but they do include a requirement that "an offer of services in accordance with an IEP is made to parents within a reasonable period of time from the agency's receipt of parent consent to an initial evaluation" [34 CFR 300.343(b)(1)]. A discussion note attached to this proposed regulation states that "for most children, it would be reasonable to expect that a public agency offer services in accordance with an IEP within 60 days of receipt of parent consent to initial evaluation." If the proposed regulations are adopted, at least this comment will help to create a presumption that, unless there are unusual circumstances, school systems must complete evaluations and present an IEP (or finding of no special educational needs) within 60 days. Notice that these proposed regulations do not speak of "school working days" and would be interpreted to mean "calendar days."
Courts interpreting IDEA have held that unreasonable delays in evaluating or reevaluating a student with special educational needs violate IDEA. In W.B v. Matula, 67 F.3d 484 (3d Cir. 1995), for example, the Court interpreted IDEA as having an implied requirement that school systems' obligations to identify and evaluate children with special needs be completed in a "reasonable time." In that case the school was found in violation where it failed to evaluate a student until April when they received the parents' request in December.
One other observation on your circumstances: You report that a teacher commented on your son's "lack of focus" and an educational aide working with him has asked for help developing strategies to work with him. Remember that it is not only the parents' job to refer for an evaluation. In this case, I would think that the teacher and/or the aide should have requested an assessment themselves. Now that you have requested it, they should be urging the school's evaluator(s) to get it done.
Also, since your son is already on an IEP, the school must provide him with appropriate services and reasonable accommodations on an ongoing basis. The aide's request for strategies indicates the program is not entirely appropriate at this time. It would be appropriate for the school psychologist to propose some strategies to improve your son's focus even before a formal evaluation has been conducted in this case. If so, you should be told what the strategies are and given a chance to object to any you consider inappropriate.