To add a little more detail on the receiver side and how it's used;
The localiser and glideslope signals are transmitted on different frequencies (the "carrier" frequency) and these are filtered out to leave a mix of 90Hz and 150Hz for each one. The 90 and 150 are then filtered out separately.
If you have more 90Hz than 150Hz, you are too far left on the localiser or, too high on the glideslope. The guidance indicator is just a display of the ratio between 90Hz and 150Hz. A 50% ratio means just right!
On your question of "for how long?", a common procedure (it might be different for a particular field or approach) is to fly towards the centre line of the localiser and below the glideslope at an angle of about 30 degrees to intercept the localiser at about 10 miles out.
ATC will vector the aircraf to this point (e.g.
Speedbird 93, descend and maintain 1500 feet, turn left heading 300 degrees. Cleared ILS approach 27L report established). The autopilot is usually in approach mode meaning that it will capture the localiser, and in turn the glideslope, and follow the ILS down.
When the localiser is captured, the autopilot will turn the aircraft onto the centrline and the pilot reports established back to ATC.
It is normal procedure to capture the localiser at a height lower than the glideslope at the point of capture so that you capture the glideslope from below. This is for safety. If you captured the glideslope from above, then the aircraft may well descend through the glideslope then level out to recapture. Descending below the glideslope, , at what might well be hundreds of feet per minute, is not good.
Finally, most procedures call for a stabilised approach (correctly configured, following the ILS, speed correct, check lists complete, no large deviations in course, height or power settings) by 1000' or go around. Many pilots, depending on their airlines procedures and conditions, will disconnect the autopilot for the last 1000 feet, sometimes to give practice to the first officer. Some aircraft are cleared all the way down (autoland) but that will also depend on company operations, crew currency and local procedures.
I deliberately left out the harmonics. I figured I only had one page ;)