It is already established that is is possible to calculate the estimated print duration by this anwer.
The most accurate estimation is obtained with specific settings for the printer. So if you have access to the printer, your most accurate estimation would be using the software from the manufacturer or correctly setup third party software. Going to the library twice might work out as you have one day to tweak the print to get it done in 4 hours, the other day to print the file generated the day earlier. Note that there are also community solutions (cheaper than commercial printer services) available where you select the most nearby printer and let somebody else print it for you.
As an alternative (to try at home), if you navigate to the LulzBot website you can find a specific release for the LulzBot printers e.g. download here and use their released profiles (from this location) to slice your print using the material to print the object in. Note that these slicing parameter files are for the TAZ, the slicing parameters can be applied to the mini. Note that this will cause an inaccuracy for the print time, but will serve as a good start.
From their website you can read that:
How to Download and Use Print Profiles
To get printing right away, the default LulzBot TAZ print profiles listed below can be imported into Cura LulzBot Edition for ready-to-print settings. To download and save the configuration file, right click and select Save As.
In order to use the print profiles below you will need to switch to the Full Settings view (Expert > Switch to full settings). After downloading, import the Cura print profile by selecting, in Cura, File > Open Profile. Navigate and select the pre-set Cura print profile file of choice.