sof2flash

It seems that you use the Web Edition and that you don't have a valid license.

Thats the reason why you have a time limited sof file.

Without a valid license you can't generate files that can be used to program your design into EPCS or flash devices. so you won't be able to generate files that can be used for production purposes.

time limited means that you need a connection between your target fpga and quartus e.g. via jtag. some time limited ip's will run without such a connection up to 1 hour.

one thing you could try with quartus 9.1 is, if your nios is not an -e type change it to -e if that is the only one component you have no license for. nios -e is free with quartus 9.1 and does not require a license. then you should be able to generate your flash files.