First ever grow: How often change reservior nft?

This is my first ever grow, the ladies have been in my nft tray for 5 days now,

NFT TANK 110CM X 50CM
5 PLANTS
50MILL CAPACITY - 34 LITRES IN IT

I HAVE NO MONEY FOR A PPM METER, BUT I HAVE A PH METER

I ADDED NUTES 1/4 STRENGTH

WHEN SHOULD I CHANGE OR ADD MORE NUTES TO THE TRAY?

hello
lol

Changing it completely once a week

but you need to top your res everyday with PH'd water the same nute concentration.

I would make batches of nuted water double what your res can hold so you can keep the nuted water to add for the rest of the week.
You have to re PH whatever you add daily however.

Finally save and go get a EFFING PPM meter. I dont grow hydo but im pretty sure things will be much easier with one
http://secure.sciencecompany.com/HM-Digital-Economy-TDS-Meter-Meter-TDS-EZ-P16349C679.aspx $16 +S&H
good from 0-9990PPM