Bilayer components will 'flip-flop' at measurable rates, but these are very different for different lipid classes. Here are the results of an experiment using fluorescently-labelled analogues.
Bai, JN and Pagano, RE (1997) Measurement of spontaneous transfer and transbilayer movement of BODIPY-labeled lipids in lipid vesicles. Biochemistry 36:8840-8848 DOI: 10.1021/bi970145r
The authors followed the transbilayer (flip-flop) and interbilayer movement of fluorescently-labelled analogues of various membrane lipids: sphingomyelin (C-5-DMB-SM), ceramide (C-5-DMB-Cer), phosphatidylcholine (C-5-DMB-PC) and diacylglycerol (C-5-DMB-DAG) in a system of unilamellar vesicles consisting of 1-palmitoyl-2-oleoyl phosphatidylcholine.
The results were analysed in terms of a model in which the labelled molecules could move between vesicles and between the two monolayers of the bilayer.
interbilayer transbilayer (flip-flop)
C-5-DMB-SM >21 s 3.3 h
C-5-DMB-Cer 350 s 22 min
C-5-DMB-PC 400 s 7.5 h
C-5-DMB-DAG 100 h 70 ms
So, for the phospholipid tested the half-time for flip-flop was 7.5 hours.