There is no industry standard AFAIK.
It depends what are you doing.
Sending e-mail just short messages withou out
large attachments a 64Kbit line might be more
than needed. Also How fast needs the mail to go
out or come in....
Is it a problem if a mail transfer of 10Mbyte
takes 30 minutes or not...
Are you running a WebServer with lots of visitors
maybe a 100Mbit line is too small than.
For average Webbrowsing & FTP transfers
(Not P2P continuesly) A 256Kbit line might be
quite sufficient.
Do you need to download ISO images a lot 1 or 2
Mbit line might be better suited...
It really depends, otherwise look for an ISP that
will allow you to upgrade a line to a higher
speed (if possible) if it appears to be to small
in bandwidth.
HIH,
Kind regards,
Nico Baggus