filler for hair? Help me out, please!!

I `m having lowlights done in a few weeks. I`ve been coloring my hair extra light ash blonde for about 4 years now. I don`t use bleach on it, but I did have some really light blonde streaks put in last March with bleach ( a year ago). WHen I get my lowlights ( of my natural color, maybe a light brown) will they need to use a filler? How do you use a filler and still do foils? Should I ask her to use a filler? How do I know if I need a filler? I just don`t want to spend alot of money and have them fade out right away. How often will I need a retouch?