Hi larder. Welcome pilgrim.
Never made wine from Welch's anything but I guess my question is why the addition of water? Are you using a concentrate? But that said, most wine makers use an hydrometer to check the gravity of their wines. The gravity (apologies for repeating something that you may know already) is a measure of the density of the liquid and the density of the wine is a measure of the amount of residual sugar in the liquor. In other words, if your liquid starts off with a gravity - of say - 1.090 (the equivalent of about 2.25 lbs of sugar in a gallon of water) then after you pitch the yeast that gravity should drop to below 1.000 telling you that all the sugar has been converted to alcohol and CO2.
While the yeast really needs oxygen to do its job, the taste of the wine benefits from you preventing oxygen from coming into contact with the wine (yeast vs wine) so wine makers compromise: during the first few days you allow the yeast the oxygen it needs and then you clamp down and prevent O2 from getting in contact with the wine. You do that when the gravity has fallen to about 1.010 - 1.005 . That is when many winemakers will rack their wine into a secondary vessel - one with no headroom and where they can bang in a bung and airlock. Primary fermentation vessels are often buckets covered loosely so that there is no problem with foam and blow-offs and the like.
If you don't have an hydrometer, let me urge you to get one. It is perhaps the most important tool in the wine maker's tool box. They cost around $7.00. They break easily...
I've always wondered why they don't make 'em out of food-grade clear plastic of some sort - guess there'd be no repeat sales...heh
