That is a loaded question. The ppm is the amount of dissolved solids (soluble elements) in a solution. If you are starting with straight water that has very little natural elements in it, and then you add your nutrients to the water, the ppm will be a measure of the concentration of those nutrients in the amount of water that you have. If you put 20ml of nutrients into a gallon of water then the ppm will read fairly high (depending on what kinds of nutrients used) but if you put that same 20ml of nutrients into 5 gallons of water then the ppm will read far lower. That is because the 20ml of nutrients is spreading out into a larger amount of water (the concentration is lower)
Now if I haven't confused you (or you already know what I just said), I generally will start new seedlings out with no nutrients for the first couple weeks until they have developed their roots good (I watch the little round "seed leaves" for yellowing before I begin feeding). Then I begin with nutes around 300ppm for the first week(plants already 2-3weeks old). If they look ok at the end of the first week, I will bump them up to about 450ppm the second week. If the plants really take off growing and looking good, I will bump them again at the beginning of the 3rd week to about 600ppm. I usually will continue to raise the ppm until I get to about 700-800ppm for most plants.
Some plants are very light eaters and you have to watch them carefully for nute burn, which will show up in a couple ways. You may see the leaves turn dark green and then curl under like a bird claw, or it may go straight to the tips of the leaves, looking like someone held a small flame to them, turning them brown. If you see this begin to happen then back off the nutrients by ~150ppm at the next feeding and see if the burn continues or stops. Or if the "bird-claw" and dark green leaves goes away in about 10 days, (this time can vary depending on the medium you are using) If you don't see improvement within 10 days or see them continue to get worse, then cut back another ~150ppm and see iff that improves them.
Some plants are nutrient hogs and will take all that you give them, but they will grow like mad and iff you are growing indoors, they will take over your grow area and have to be cut back continuously until you get them to flowering.
I generally will increase the nutrients for flowering as the plant almost always wants more for flower building. I generally will bump up my "bloom" nutrients(each week) while reducing my "veg" nutrients(by half each week) until I get to about 1100-1400ppm with bloom and micro nutes, but the veg nutes are cut out completely. Some growers will push their plants as hard as they will go, which is to raise the ppm until they get a little burn then back off some. I will do similar but I don't push mine as hard depending on the strain.
Then when I get to the last 2 weeks of flowering (if everything has gone right, with no problems), I will cut the nutrients in half for one week then only straight water for the last week. This is both to allow the plants to use up all of the available nutrients within the plants and system, and to conserve the nutes as in the last couple weeks, the plants don't use as much as they are winding down. This last part can be a little tricky to do because unless you really know the plants well enough to say you know that they will be ready at a specific time, you may end up going longer than you expect.
I hope this is understandable and helpful. If you need any help don't be afraid to ask.