When is the best time to test PH levels in a wine started with fruit and water?

I've read not to trust the amount of Acid blend asked for in recipies. I should be measuring the PH level in the consentrate or juice. Well I don't get juice till I'm in at least half way in the formentation. Can I measure it then or wait till the fermentation is done? If I do it the middle of the fermentation, and add the proper amount then, will it possibly stop the yeast?

You can add acid any time really, but the sooner you do the more integrated it will be in the wine.
I don't know what an acid blend is sorry. I am hoping it is not what the name suggests - a blend of tartaric, malic and citric acid. In 99% of cases you should only be using tartaric acid. Citric acid is sometimes used by winemakers before bottling in light floral white wines to give it a zesty fresh character. Malic is used in some situations, but I wouldn't risk it in home winemaking. Also, by using straight tartaric acid, you can pretty much get the pH you want because you know that 1g/l of tartaric acid will drop the pH by 0.13

Why don't you have juice until halfway through fermentation? You can't measure pH during fermentation without degassing the wine (CO2 interferes with the meter's probe), so really you should have tested it on the juice before inoculation with yeast.
You can always add it after primary fermentation though. My normal protocol is some before primary fermentation, if required, and then some before secondary (amount depending on pH and TA). After that I try to avoid it.
But to answer your question, I wouldn't be adding it during fermentation - wait til you have racked the wine of gross lees following the primary ferment. Aim for a pH of no more than 3.6
and no less than 3.4 in reds, in whites I would be aiming for 3.3 to 3.5. Test the pH 2 weeks following addition. Then in another 3 months. But if you add the right amount early on you shouldn't have to add it again.

Thanks OsWine. Thats what I needed to know. You are confused why I don't have juice? Well I use fresh fruit. There is no juice in the bulk of fresh juice unless I squish a sufficent amount out of the fruit. I might not need much for PH strips but I got the impresion I might need a descent amount for an acid test kit. These acid kits are not available in my area, and they look at me funny when I ask about them. These local store sell wine juices and I prefer making my wine from fresh fruit.I'm from Canada and all I can find are stores from the states on the net.So I am inquiring before I check out our big cities.Thanks again.

In Australia, we can get a half decent pH probe for AU$80 (maybe US$60). It is accurate to +-0.05 pH units. If you are willing to spend AU$200+ they can be as accurate as +-0.005 (I really recommend this... it is a seriously good investment). You will need to buy some buffers to calibrate it. These come as red and green liquids and are cheap as chips. Maybe look on ebay or some online store. Search google.

With the fresh fruit... you really need to squish it as much as you can to release the juice and blend it all together. Sieve 50ml or so of the juice into a beaker/small glass to measure pH. If you don't want to do this, as I said, you can always wait until ferment is finished. That is not really a problem.
But whatever you do, make sure the pH is right before bottling.
Cheers.... and Merry Xmas of course