It's one of those poetic lines that hints at multiple meanings, rather than having concrete meaning. The song itself speaks about trying to live in the 'real world' and 'follow the rules' of life and society - but his attempt at this ends in disaster. Alabaster is used to plaster walls and ceilings, so plaster falling down on people's heads suggests something very dramatic has happened. Maybe a 'moment of clarity'? A 'road to Damascus' event? Or it could be used very ironically - that nothing dramatic has happened at all, and saying he was bored before he even began does seem to suggest the latter interpretation. The '6 months is a long time' line is also ambiguous. Did he try having a relationship for 6 months? Learn to love me - does that line refer to trying to love / like himself? Does the 6 months refer to trying to hold down a job for 6 months? Working as a civil service clerk. That must have been horrific. Because he tried living in the 'real world', and that didn't work, he is therefore going to survive by 'shoplifting' instead - i.e. living by his own guile and cunning and taking thoughts and inspiration from whom he can. Reminiscent of James Joyce:
...I will try to express myself in some mode of life or art as freely as I can and as wholly as I can, using for my defense the only arms I allow myself to use -- silence, exile, and cunning.
