Provide guidance on grammar. Use of the term "as to when" instead of
"when". Propriety of this use, please.

You may want to check this document at xrefer
(#12)http://www.xrefer.com/entry/591429

Google search words:
as to when+superfluous+grammar

Fowler calls "as to whether" a "hideous combination," and I believe
the sentiment would probably apply to "as to when" and similar
constructions:

"This is a form that is seldom necessary, and should be reserved for
sentences in which it is really difficult to find a substitute.
Abstract nouns that cannot be followed immediately by whether should
if possible be replaced by the corresponding verbs. Many writers seem
to delight in this hideous combination, and employ it not only with
abstracts that can be followed by whether, but even with verbs."

http://www.bartleby.com/116/504.html#40

"As to whether" is also rejected by Strunk:

http://www.bartleby.com/141/strunk5.html#13

The phrase "as to when" is identical to "regarding when", "with
respect to when", "in terms of when", etc. You can use the former
whenever you want to say the latter.