The default is 250ms. I just did a quick test by changing it to 3000, and it waited three seconds before displaying.
Are you sure you are seeing Auto-Complete, and not Auto List Members?
In the case of Auto-List members, when that is enabled, it is always displayed immediately after you type the member access operator. However, if you *disable* Auto-list members and leave Auto-Complete enabled, you can still get list-members but it will only be displayed after the Auto-complete delay *and* after you type part of the identifier.
Example:
struct SOME_STRUCT { int m_xxx, m_yyy, m_zzz, m_ttt; };
void nothing(SOME_STRUCT *px)
{
px-> // Auto-list members would normally come up immediately after you type ">" here if enabled
;
px-> // Auto-list members will not come up if it is not enabled
// Auto-Complete will not come up either as you have not typed any part of an identifier.
;
px->m // After you type 'm', Auto-Complete will come up after it's configured delay if enabled
}