Days and days I suspect, as every possible 'edit', 'add' and 'delete' would have to be 'backed up' as it were. Saving the level every click would also be a mess. The work to do ONE undo and a hundred is much the same, you just multiply out the memory requirement. In classic, we had a 'sort of' undo which most users where not too happy about so it makes sense to code it properly when the time comes
PC SPECS: Windows 7 Ultimate 64-bit, Intel Core i7 920 (PASSMARK:5008), NVIDIA Geforce 9600 GT GPU (PASSMARK:752) , 6GB RAM