What kind of problems can be faced if any session which connects to unix server is open for longer time?
How to find out from how much time that session is idle? 
Simply type w
ant:/sm/doc $ w
10:33am up 125 days, 25 mins, 20 users, load average: 0.00, 0.00, 0.01
User tty login@ idle JCPU PCPU what
vbe pts/0 4:10pm138:23 110:09 110:09 gpm
vbe pts/1 4:38pm112:42 ssh lo4
vbe pts/2 4:48pm905:43 rlogin rh5c
vbe pts/3 5:49pm 1 /usr/bin/ksh
vbe pts/4 5:44pm428:08 rlogin n12
vbe pts/5 6:11pm571:04 1 1 rlogin n12
vbe pts/6 10:35am143:56 rlogin sciv
vbe dtremote 4:01pm 7:47 -
vbe pts/7 10:39am450:08 rlogin n12
...