I am not sure when I should harvest this lettuce it is already 30 days since I planted the seeds.
What does it say on the seed package? Typical seed to harvest for lettuce is about 6 weeks in hydro. But there are many varieties, and they don't all grow at the same speed.

So how am I suppposed to know when to harvest I am still seeing new leaves coming out and some of the leaves are measuring 9 inches long and 6 1/2 inches wide at the widest part.
If your waiting for them to stop growing before you harvest. Well, they wont stop gowning until they die. Again refer to the seed package. There are thousands of types of lettuce. Shape and size vary's greatly, and harvesting time is up to the grower.

Since this is my first crop I will be using a ph and ec meter soon since I ordered them from the States,but I have been changing the nutrients out after a week.
That's fine if you have the money to spare. But EC/TDS/PPM meters are not worth the money. pH is very important, but I wouldn't waist money on a meter. Meters are electronic devices that can give false readings. But pH drops are cheaper than the calibration fluid needed for meters, and wont give false readings, or even need special care like electric like meters.

I am using 70 percent shade cloth is this too much? .
That depends on the situation. If the sun is strong, and temps are high. I wouldn't use less. But a big factor in how tight the leaves are is temperature (not necessarily sunlight). Lettuce is a cool weather plant. It will tend to elongate and get ready to seed when temperatures are warm. Shade cloth can slow that down to a point, but the actual air and water temperatures are a huge factor in when it will begin to seed.