In the past two weeks, I have seen seven plays, a ballet and an opera. Though it is less than some of my critical colleagues, it feels a little much and has caused my family to moan about my perpetual absence from home in the evenings.

But it has put me in a mood to consider my own new law of theatrical relativity – of how long things can feel short and short ones interminably long. For example, I saw Heisenberg - The Uncertainty Principle and Oslo on successive nights. Simon Stephens' new play, despite its title, isn't about physics at all: it is about the relationship between two people and it comes in at around the fashionable 80 minutes (without interval). Short and sweet you might think, yet at around the 60 minute mark, it suddenly seemed to drag because the subject matter felt too slender to sustain all the effort being expended on it.

Oslo, on the other hand, JT Rogers magnificent epic about the forging of the peace accord between Israel and Palestine in 1993, positively sped by. Its length should not deter anyone from going to see it at the Harold Pinter Theatre where it runs until the end of December. Despite its heavy-weight subject matter, it has the deftness and flow of a much shorter, lighter piece while never skimping on seriousness. I could have watched it for another three hours, it is so sleekly built.

I rather mourn the old-fashioned two and a half hour play

It's the writing and the production that carries you through an evening at the theatre. If you're engaged and engrossed, then time flies. Robert Icke's recent production of Hamlet used a very full version of the text, yet its intelligence and the interest it provoked made its length irrelevant. I remember, in contrast, a Hamlet starring Alan Rickman at the Lyric Hammersmith many years ago which was so ponderously intense, time actually seemed to stand still, much though I loved both actor and play.

I rather mourn the relative rarity of the old-fashioned two and a half hour play. Theatrical experiences nowadays seem to either be 90 minutes straight through (excellent for dinner) or a thunderous three hours which makes eating tricky but which can be nourishing in itself.

For all my reservations about Dominic Dromgoole's revival of Oscar Wilde's A Woman of No Importance, I did enjoy its shape: four brisk acts, clearly delineated, separated by a half-way interval, with curtain down after 180 minutes. I love that in Rattigan as well. And Pinter, whose actor's background gave him an innate sense of theatrical structure. Brevity is not always the soul of wit, but it shouldn't be despised. You don't have to write at the length of George Bernard Shaw or Shakespeare, to prove you have something important to say.

Novelists are always advised to "kill their darlings" and it is a useful lesson for aspiring playwrights as well

For example, I couldn't help thinking that Mike Bartlett's Albion, which I adored when I saw it at the Almeida, would have been better for being shorter. Perhaps a little pruning by a sub-editor or a director would have sharpened and strengthened an exceptional piece.

Other plays are in need of much more radical surgery. Rory Mullarkey's Saint George and the Dragon, which has just opened at the Olivier Theatre, and also runs at almost three hours, feels desperately in need of cutting. Novelists are always advised to "kill their darlings" – sacrifice the passages they most love – and it is probably a useful lesson for aspiring playwrights as well. However hard it feels at the time.

Theatre has much more flexibility than dance when it comes to timekeeping. The choreographer Mark Morris told me years ago that there was a danger that all ballets end up being around 30 minutes, so they slip neatly into a triple bill or full-length so the audience feels it has had its money's worth. That remains broadly true; many a dance piece that should last a maximum of 45 minutes is fatally extended to more than an hour because otherwise it is too difficult to sell.

But playwrights sometimes fall into the same trap; either an idea isn't fully developed because they are trying to keep things tight, or they can't stop themselves exploring every twist and turn of their ideas. My rule of theatrical relativity suggests that only if you find a length that is precisely matched to the richness of your content will an audience – or at least this member of the audience – be entirely satisfied.