John, the use of the expression “donkeys” on it’s own is unusual in UK. The usual term is “donkeys years”, meaning (as you know) a long time. The use of the word “donkeys” is probably rhyming slang for “donkeys ears” (years). For some reason, although you would expect the word “donkeys” to be sufficient on it’s own, the word “years” is almost always added. Having said that, generally, your English is very good.