...or read doc/TRICKS

 - If you don't want your logfiles to be deleted after two days and don't    want the bot to create a new logfile each new day, then set 'keep-all-logs'    to 0 and 'switch-logfiles-at' to 2500 in your bot's config file to make it    keeping one logfile all the time. This is not recommended on high traffic    channels.

set max-logs 5
set max-logsize 0
set quick-logs 0

logfile mbco * "logs/lame.log"
logfile jpk #lame "stats/lame.log"

set log-time 1
set keep-all-logs 1
set logfile-suffix ".%d%b%Y"
set switch-logfiles-at 300
set quiet-save 1

This is better in my opinion, eventualy the file will become very large and you'll need to delete it. Thus, I find having a separate logfile for each day is better; anyways, if you want it to log into only one file then I'm sure a tcl script can do this task very well.

##### LOG FILES #####

set max-logs 5
set max-logsize 0
set quick-logs 0

# This creates a logfile named eggdrop.log containing private msgs/ctcps,
# commands, errors, and misc. info from any channel.
logfile mbco * "logs/botnick.log"

# This creates a logfile named lamest.log containing joins, parts,
# netsplits, kicks, bans, mode changes, and public chat on the
# channel #lamest.
logfile jpk #lamest "logs/lamest.log"

set log-time 1
set keep-all-logs 0
set logfile-suffix ".%d%b%Y"
set switch-logfiles-at 300
set quiet-save 1

# If you want to keep your logfiles forever, turn this setting on. All
# logfiles will get suffix ".[day, 2 digits][month, 3 letters][year, 4 digits]".
# Note that your quota/hard-disk might be filled by this, so check your
# logfiles often and download them.
set keep-all-logs 0

# If keep-all-logs is 1, this setting will define the suffix of the logfiles.
# The default will result in a suffix like "04May2000". "%Y%m%d" will produce
# the often used yyyymmdd format. Read the strftime manpages for more options.
# NOTE: On systems which don't support strftime, the default format will
# be used _always_.
set logfile-suffix ".%d%b%Y"

# You can specify when Eggdrop should switch logfiles and start fresh. You
# must use military time for this setting. 300 is the default, and describes
# 03:00 (AM).
set switch-logfiles-at 300