There isn't a specific amount - you should give her enough food that there's a small amount left over at the end of the day. That way you're sure that she got all she wanted but there's not enough to feed a family of chicks.

You could try giving her the 'several days of neverending light' treatment sometime before the expected hatch date of her current clutch. There's no harm in her sitting on these eggs, but you don't want her to lay a new clutch when the current eggs fail to hatch.

She's been sitting since January 29th. This is 22 days on 5 eggs. Is this a normal amount of time? When should she give up on them?

Emma should give up on her eggs around a couple more days.

I agree, she will probaby give up soon. The oldest eggs are past their due date, but if these were fertile eggs there would still be a chance for the younger ones to hatch.

Emma gave up on her eggs about three days ago.

Hooray!

Nice! Here's to hoping she won't lay another clutch.

It was 2/26/15 when Emma gave up on the last group of eggs. She layed one egg on Monday, 6/8/16. She is spending some time sitting on it.

I've noticed her masturbating quite a bit over the last week or so.

Is this a normal interval?

Just about any interval is normal with cockatiels. They're opportunistic breeders who will go for it any time conditions seem right.

Emma has four eggs now. She spends all her time sitting on them. She's very defensive of the area where the dish with the eggs in it is. She won't let Silver on the bottom of the cage at all. She lunges at me when I try to change out the bottom of the cage for cleaning.

All this seems normal. I don't know why she thought the conditions were right for laying. I was keeping the nights long; she wasn't getting much soft food; she wasn't mating with anyone; I was keeping her out of nesting places.