Hi, that depends on many additional things, hardware side: your RAM speed, CPU, memory centre and software side: your operating system. lighting settings and especially the size of your project. It would be difficult to find an exact figure but building your lighting is a CPU intensive process which requires a lot of memory. 8GB of memory, and I’m assuming you have a quad core processor, is minimum recommended specification. It probably will take a very long time to build in any case unfortunately. The crashes would suggest you are running out of memory which means you have reached the maximum amount of unbuilt lighting your 8GB of RAM can handle. You could speed up your build by optimising your light-map resolutions and general lighting. It is very likely you have these settings set higher than you need but if you want the extra resolution you have to wait for it. Ultimately, if you are working on large projects, I would recommend investing in a more powerful computer. The increased performance will make a huge difference on your build times.