How much rain is too much? To people that are suffering from dehydration or areas that have been in drought for too long, they would probably answer with an open ended statement suggesting that there could never be enough. But to anyone that has been subjected to floods and their effects, it is a serious question that requires a careful answer.
Too much rain generally is when an area has reached above the normal height of its water table. In this respect the aquifers in the ground are over-saturated and streams, rivers, and lakes will rise in response. With more rain, there is greater saturation and more water than the land can accept. This over saturation is the root of sustained flooding, which will allow water to spread out in a horizontal direction until it is restricted by levels of saturation and aquifers in other geographical locations.
With sustained flooding, water will remain in place until the downstream flow draws enough water away from a saturated area and the outer water tables can leech it out. This type of flooding differs from flash floods because of saturation. In typical flash flood scenarios, too much rain or other collections of water assault the surface of ground too rapidly to be absorbed and flows on top. For this reason, water tables, aquifers, and saturation levels can be low and still result in flood conditions until the ground absorbs the excess water.
Aside from just floods, too much rain can be slight and steady quantities that are absorbed and channeled away by water systems, but can still have a negative effect on plants in the region. As plants need both air and water absorbed into their root systems, some plants can die from drowning. Areas with poor soil drainage are especially at risk, and some soils that hold too much water but lack the ability to hold the land, resulting in mudslides.
Excessive rain can also affect local summer insect populations, turning standing water from unending rains into perfect breeding grounds for mosquito larva and other organisms. Bacteria in swamps and stagnant water also pose as hazards for drinking water if they merge into the aquifers. Both these would not be possible if just enough rain fell.
Too much rain is too problematic, but it always depends on the region involved so there is no specific numerical answer. Besides simply climate, elevation is another factor, where drainage and the Earth's ability to move the water factor into the equation.