One is not allowed to partake of any food or drink before reciting or hearing the kiddush of the day. However, because coffee helps with concentration during prayers, there's a leniency to allow one to drink coffee before the morning prayer.
Until when does this leniency extend? Is it only before the beginning of shacharit? What about before mussaf or during the Torah reading?
One should not taste any food before the morning kiddush (SA OC 289.1). The reason one can drink water before praying is that there is no obligation of kiddush before praying and water is not considered as food in this context.
Therefore you can drink water until you finish praying shacharit (i.e., shmone esrei). Then the obligation of kiddush starts.
As to drinking during shaharit (since you ask when exactly does the leniency end): technically one cannot interrupt between psukei dzimrah and shmonei esrei (SA OC 51.4) so it appears one can drink until right before starting psukei dzimrah.
Note one cannot make a full meal until one has prayed Mussaf but fruit or less than 54 grams or cake is allowed (SA OC 286.3).
And if you need a practical ruling CYLOR as there are leniencies for e.g., sick or older people. It is also possible one could even drink during psukei dzimrah if one has already said sheakol as the SA above discusses speaking as an interruption.
According to this Star-K article
On Shabbos morning, men may drink water, tea or coffee before Shachris (after brochos), but may not eat and may not drink “chashuva beverages” (e.g. alcoholic beverages) unless they are required for health purposes. After Shachris, one may not eat or drink until after Kiddush.
A woman who normally davens may eat or drink before davening after reciting morning brochos. According to some Poskim,42 on Shabbos if she needs to eat before davening, she is not required to recite Kiddush at that time. Once she has completed davening Shachris, she must hear Kiddush before eating or drinking.
My understanding of the above is that you can have coffee as long as you haven't started Shacharit, and you would have to say the "birkot hashachar" first, anyway. Once you have past that point, you have to wait for Kiddush, if you're male.
FWIW, this would explain the "schnapps clubs" in many shuls, where people make Kiddush between Shacharit and Musaph. (Many shuls in my neighborhood, have a no alcohol policy which includes no wine, as well. Kiddush in shul is said on grape juice.)
