assuming your QV-Server is QV11 and is installed into the standard-path, you should have a look at the file "C:\ProgramData\QlikTech\DistributionService\Configuration.xml" - there you should be an option like the marked one in my screenshot:
I didn't test changing the value, so I don't know if it works, but I would think so. Make sure to backup the original file and stop the distribution service before changing values in your config files. Also you should be very careful in clustered installations.
I would also recommend to watch the size of your log when you change the mentioned value (I wouldn't set it to 9999!), especially when you have normal or debug logging and a lot of tasks.
This is not an option in Version 10 or really version 11. If you change the Configuration.xml file and make changes in the QEMC In the distribution Services it will over right overwrite the file to the default of 30. To see this change the file and then change the QV Engine number and you will see that it has changed.
I believe it was change to a default setting after version 8.5.