There is no set time as everyone is different. Some cant get ewcm outside their body but must look inside ( i know...
) while other gets tons when wiping and on pants.
Others only get it on ov day while others start having it 5-6 days before ov (like me) and even a few days after.
You simply need to monitor and find what is normal for you and then you will get a picture how your body works. Keep in mind that its the cervix that produces the ewcm (the clear, or sometimes slightly cloudy stretchy stuff, hence the name eggwhite, cause its similar in appearance and texture) and depending on how long and how much oestrogen you body produces it will vary from woman to woman.
There are things you can do to make the ewcm more abundant and it really does work. But unless you feel really dry when bd'ing etc its not strictly necessary to have boatloads of the stuff to get pg, although it is there to nourish and string the sperms along the vagina up through the cervix.
Usually ewcm means you are in your fertile window and it certainly doesn't hurt to get busy as soon as you see it - i obviousy don't as i get it quite a while in advance - i.e. when you get ewcm you are about to ov, however, without monitoring your cm and more importantly your ov, you have no real way of knowing when you actual are ov'ing. But hey, if you have the energy, a little extra nookie never hurt anyone, lol!!
Hope that helps!