I use the following script (powershell) when I need to do this:
$doforever = 1
$cn2 = new-object system.data.SqlClient.SQLConnection("Data Source=somedatasource;Integrated Security=false;Initial Catalog=master;User ID=sa;Password=somepasswordimnottelling");
$cmd = new-object system.data.sqlclient.sqlcommand("dbcc sqlperf(logspace)", $cn2);
$cn2.Open();
do
{
$ds=New-Object system.Data.DataSet
$da=New-Object system.Data.SqlClient.SqlDataAdapter($cmd)
[void]$da.fill($ds)
$ds.tables[0] | out-string | %{$_.split("`n")}| %{$_ -replace("\s+",",")}|%{"{0},{1}" -f (get-date -format 'dd/MM/yyyy HH:mm:ss'),$_} | add-content -path c:\perflogs\admin\logsize.csv
sleep -s 300
}
while ($doForever -eq 1)
Quick and dirty - I know it can be refined :)