where can i find settings for wifi internal scan period
This property comes from during build-time and is extremely, dependant on the wifi driver, for each and every device tree that gets integrated into the build of Android, there's what's called an overlay, this overlay may contain extras pertaining to the said device in question. 

In fact, its rather, an XML document that gets parsed during build and "overlays" or "overwrites" the standard Android defaults, such as animation speed, etc.

This pertinent information of this comes from the device's overlay/frameworks/base/core/res/res/values/config.xml which contains the very exact line:

&lt;!-- Boolean indicating whether the wifi chipset has background scan support --&gt;
&lt;bool translatable="false" name="config_wifi_background_scan_support"&gt;true&lt;/bool&gt;


From that, that is telling the build, to say, that the property wifi.supplicant_scan_interval is supported and can be used dynamically from the build.prop file found in the /system directory, which in turn, is read from the Android's native binary executable wpa_supplicant. 

However, it must be said, it depends on the Wifi Driver to support that, and how much integration of the Wifi functionality is within the frameworks. To find the exact source can be found in external/wpa_supplicant/wpa_supplicant.c, around lines 2459:

#ifdef ANDROID
    char scan_prop[PROPERTY_VALUE_MAX];
    char *endp;
    if (property_get("wifi.supplicant_scan_interval", scan_prop, "5") != 0) {
        wpa_s-&gt;scan_interval = (int)strtol(scan_prop, &amp;endp, 0);
        if (endp == scan_prop) {
            wpa_s-&gt;scan_interval = 5;
        }
    }
#endif


default here on Gingerbread is 5secs, again the mileage will vary depending on the version of wpa_supplicant used, there was two versions, wpa_supplicant_6 which was broken under ICS, and the newer version wpa_supplicant_8, despite saying that, it may no longer be supported now with the newer releases.