I had this idea of a running sketch where you see the first part and then the rest is spaced out between other sketches to signify time passing by.

INT. - TOILET - DAY

A man suffering from constipation sits on the toilet trying to go but without much success.

MAN
(straining)
Arrgh!

He then gives up frustrated and cuts to another sketch.

Returning again

MAN
(more straining)
Oh come on!

Cut to another sketch

Return various times to see him

reading the full set of Britannia encyclopaedias

building a castle out of matchsticks

doing a massive jigsaw puzzle

having made origami figures of Noah's ark and all the animals

Fed up now, he's red faced with sweat pouring out of him but still no joy. Then with a facial close up, blood trickles from his nose just as he tries one more last massive effort.

MAN
Yerughhhhhhh

Until finally he goes into a shocked look, falls off the toilet onto the floor, with a down the loo close up camera shot showing his heart beating away in the pan.

I like it, i would show, just before the final shot, the man taking some extra strong laxitives.

COOL!!!! Liked it a lot!
Maybe he could give birth to a baby elephant and its little face was looking back at him in the pan. Or better still my man would be huffin and a puffin and straining so much then the camera shows the pan, with a tiny rabbit dropping sized poop in it.

Quite liked this. Think the montage would work well enough for time passing to be conveyed without having to cut to other sketches.

Dan

I thought as I read it that it was going to be a bit stupid - but when I read the bit from the blood trickling out of his nose I couldn't help laughing.

For me the image was so disgusting it was hiliarious!

Possibly you could replace the heart with intenstines to make it even more gruesome. Also, you could develop a bit on the in-between scenes; the jigsaw puzzle could be of a toilet or you see him put the last matchstick on the castle but it all falls down.

Thanks for your comments everyone and many good suggestions and there are so many possibilities for something like this.

Another idea I liked was to come back after a while and show him eating a pizza he'd had delivered...or at least a discarded pizza box on the floor.