I find the chart below (published by OSHA) to be handy. For example, a chain saw generates 110 decibels of sound, so anything over 30 minutes of exposure could cause permanent hearing damage. I use a fairly cheap decibel meter to test sounds around the farm. For example, the sound level in the cab of my tractor is around 70-80 decibels, so I know I'm safe to go without ear protection. Of course, I generally use ear protection for anything "loud" even if the charts says I'm ok -- I wouldn't use a chain saw unprotected, even for 10 minutes.
I plan to get some custom molded ear plugs, but haven't yet. I've tried the generic soft plugs but have a difficult time getting a good seal sometimes and also worry about damaging my inner ear by forcing the plug in there. I also run into loud sounds elsewhere so would find them handy. For example, on an airplane, I've clocked some of the safety announcements at close to 100+ decibels. Interesting that the "safety" announcements create a different sort of safety hazard themselves (to hearing).
Sound (decibels) Hours Exposure
_________________________________________________
80................................... 32.0
81................................... 27.9
82................................... 24.3
83................................... 21.1
84................................... 18.4
85................................... 16.0
86................................... 13.9
87................................... 12.1
88................................... 10.6
89................................... 9.2
90................................... 8.0
91................................... 7.0
92................................... 6.1
93................................... 5.3
94................................... 4.6
95................................... 4.0
96................................... 3.5
97................................... 3.0
98................................... 2.6
99................................... 2.3
100.................................. 2.0
101.................................. 1.7
102.................................. 1.5
103.................................. 1.3
104.................................. 1.1
105.................................. 1.0
106.................................. 0.87
107.................................. 0.76
108.................................. 0.66
109.................................. 0.57
110.................................. 0.50
111.................................. 0.44
112.................................. 0.38
113.................................. 0.33
114.................................. 0.29
115.................................. 0.25
116.................................. 0.22
117.................................. 0.19
118.................................. 0.16
119.................................. 0.14
120.................................. 0.125
121.................................. 0.11
122.................................. 0.095
123.................................. 0.082
124.................................. 0.072
125.................................. 0.063
126.................................. 0.054
127.................................. 0.047
128.................................. 0.041
129.................................. 0.036
130.................................. 0.031
