If the drains are open and functioning properly there should never be any water standing in the drain lines. Standing water, no matter how slight, is indicating the lines are either partially clogged or improperly pitched.
There is a vast array of problems that can result in clogged drains.
Some of the most common are:
1. tree roots entering the house sewer line through a joint or crack in the pipe.
2. A cracked or broken line that has sharp shards penetrating into the flow area that catch bits of solid waste and hold them in place.
3. Lines are internally coated with cooking grease that forms a hard yellowish white sludge on the pipe walls.
4.Improperly disposing of paints, oils or other household chemicals into the drainage system. (Washing paint brushes or sheetrock mudding tools is a prime contributor here)
5. A partially obstructed vent that restricts the transfer of vent air and causes a reduction in the velocity of flow. (Reducing the velocity impedes the drains ability to carry any solid wastes that may be present in the line.)
6. A oversized drain line. When a line is properly sized the level of liquid in the line at full flow should be equal to 1/2 the pipe diameter. This leaves the upper half of the pipe open for the transfer of vent air as the liquid moves through the pipe. If a line is undersized the pipe will often be full of liquid, which then prevents the movement of vent air and restricts the proper velocity of flow. If a line is oversized the level of liquid in the line is not sufficient to carry any solids that may be present. The solids then settle to the bottom of the line and the liquids flow around them, leaving the solids which will later dry into a hard mass once the liquid and passed. In fact, an oversized line will clog much faster than an undersized line..
7. Excessive use of liquid drain openers can cause serious clogs. Most of the liquid drain openers that are available to homeowners are variations of a Lye base. Lye can combine with cooking greases to form Lye soap, which will make a solid mass that is very difficult to clean out.
The only proper way to clean a drain line is to rod it out with a snake that is equipped with cutters to match the diameter of the pipe. Using an undersized snake will only punch a small opening through the clog allowing the liquid to move, which then leaves the solids in place and as the liquid goes away the solids are now exposed to vent air and will dry into a hard mass.
I would personally caution all homeowners not to even attempt to clean a line with liquid drain cleaners. The problem is, if the liquid does not do the job you are then confronted with the task of dissassebling traps and piping that contain a potentially dangerous liquid. (I charge an additional $75 for dealing with standing water containing liquid drain openers. Why? because over the years I have had my clothing litterally fall off my body from acids, not to mention a number of serious acid or caustic burns, not something I care to repeat).
When we consider that a floor mounted powered drain auger capable of cleaning 2", 3" or 4" lines will typically cost about $600 to $2000 it is understandable that most homeowners will not have them. These type of machines can usually be rented at any local tool rental service and in some cases your local hardware or home supply center may have them available for rent for about $30 to $45 a day.
The other option is to call a professional drain cleaning service. While the service fees normally charged by a professional drain cleaning service may seem quite steep in the end they are often a bargain because they have the proper equipment and personnel properly trained in how to do the job most efficiently. Often they can do in 15 minutes what might take a novice 2 or 3 hours to complete.