if there is no HW errors, disconnects...etc everyone should have 99+% efficiency after few hour (difficulty does NOT matter!)
more hash u have mean less time to balancing out luck/unluck at higher difficulty
but there is no way to be "unlucky" all the time and getting bad efficiency without any reason!

Work Unit = shares / minute @ difficulty 1 (diff1 = 2^16 at scrypt coins)
HRA = average hash rate (kHs)

efficiency = WU * 65536 / 60 / 1000 / HRA
(*100 to get res. as %)

after 1+ week mining i (stil) have more than 99% efficiency on all rig
1h ago restarted my solo card desktop PC = 110% efficiency atm.. and will go down to ~100%