you need to wait untill it is completely dry (6 month is a good period) for small frescoes could be less, depending on how thick the plaster is.

"what would happen if I do not wait at all and add the cow bone glue and the canvas"

I would assume that glue will penetrate too far in adhering to sand in plaster and the whole thing will be ruined.

"And another question is, can I make the taking-off-the-fresco easier by adapting the intonaco layer (more sand perhaps or another kind of sand?)"

With strappo you are not removing sand, but a thin "crast-like" layer of "calcified pigment" wich later gets "glued" to the new support (old support was the oruginal plaster).

original - step-by-step topic here:

http://www.truefresco.com/dcforum/DCForumID1/72.html