I’ve let too much time pass since I saw you last.
This is the polite way to greet someone you haven’t seen in a long time, whether you happen to be visiting the person at home or meeting her elsewhere. The literal meaning of the message is that you haven’t been passing along news and information the way you should, but in conventional usage it has the effect of an implicit apology for not staying in closer touch with someone you consider to be a benefactor.
Another expression that appears quite similar in form actually has a very different usage.
Doumo goaisatsu ga okuremashite …
I really should have been in touch sooner.
This phrase is employed when a tentative business transaction has heretofore been sketched out by telephone or fax, and the party initiating the deal has appeared to make his proposal in person. By using this expression, he conveys the sense that, by all that is proper, this formality should have been observed much sooner. By way of comparison, consider two very common expressions that are used in the same sorts of situations as gobusata itashimashita:
It’s been a while.
It’s been a long time, hasn’t it?
Unlike the first two expressions introduced above, neither phrase carries any sense of apology. These are much more casual forms that—not unlike “Long time, no see”—are perfectly appropriate for greeting people you know well but probably too casual for mere acquaintances and definitely too offhand for your elders or superiors at work.
When you meet an acquaintance you haven’t seen for some time, it’s naturally considered courteous to talk about how things have been. Several standard lines are available for this purpose:
Okawari arimasen ka.
How have you been?
(literally, “Have there been any changes?)
Chitto mo okawari ni narimasen ne.
You haven’t changed a bit.
Okawari nakute kekkou desu ne.
You’re looking well, as ever.
Ogenkisou de nani yori desu.
You look well, and that’s the important thing.
The latter three expressions are, of course, logical choices when the other person actually does look well, but even if that is not the case, these words may still be appropriate. It may be, for example, that the other person has been ill or has suffered some misfortune, which you have heard about from other sources, and you may wish to cheer him up by using expressions such as these.
– Source: A handbook of common Japanese phrases –