Wet feet, they can be quite the problem. Water can be a major problem to have. Water can create mold in a house, water can rot a wooden fence, water in your boots can help lead to trench foot, and it can be bad for a plant to have too much moisture near it roots.
Some plants can handle having it roots in very moist soil, while other plants cannot handle wet feet too well. Some plants like birch trees, red maples, ferns, various irises, just for some examples can handle the moisture near their roots. Some plants that absolutely do not like wet feet are yews, white pines, witch hazel, lilac bushes, boxwoods, just to name a few of these.
Now if you have a plant that has wet feet, but they are not suppose to, then you are going to start to experience some problems. When are the problems going to start? That is hard to say. There are many factors that cause how fast or slow the effects will be. The amount of water, the soil conditions and type of soil, how much evaporation there is, how much water the plants need, etc. can all speed up or slow down the damage the plant will experience.
Don’t worry quite yet, many factors can be changed to help your drowning plant out. Watering less is a start for wet areas. If you can, aerate the soil, but make sure not to harm the root system of the plant. Alter the way the area drains excess water. Just for some potential fixes.
All living things need water, including plants. But some plants can handle water better than others. For some, too much water can do quite a bit of harm to the plant, and the plant may suffer because of it. So if you see a once healthy plant starting to become sick, and the soil around the plant is wet, I would at least start there when looking for a cause.