I agree that they should make it obvious when an offer ends for online customers, but with a bit of knowledge you can, to some extent, get round the problem.
Most of the chains have their changeovers on Wednesday, so if you are buying on Tuesday for delivery on Wednesday, say, there is a chance you'll miss out on the offer. That means that the best time to do your on line shopping is From Wednesday on, and never shop during the early part of the week for delivery after Tuesday.
Most promotions last for 4 weeks (in Co-Op stores it's 3 weeks), occasionally there are shorter promotions (one week, a weekend, while stocks last) but most of them are for 4 weeks.
Boots and Superdrug do changeovers every two weeks (half the store at a time, but the offers on each half still run for 4 weeks, if that makes sense).
If you are in the store yourself, you can check the promotion end dates easily enough if you know where to look, and what to look for - the stores don't always make it obvious when a promotion ends. The trick is to look closely at the "barkers" (the large offer cards that are usually attached to the edge of shelves). The dates covered by an offer are usually at the bottom, in tiny print, often tucked in with other coded stuff for the chain.
In Sainsbury's for example there will be two dates - the start and end dates - separated by a / whereas in Boots the dates are, if memory serves, separated by a hyphen. You still have to look closely - I think one of those two does the dates in ddmmyyy format and the other does them in yyymmdd format. Superdrug usually have an "offer ends dd/mm/yyyy" bit, but it's still pretty small print. Co-Op have something similar, but a lot of those small stores are not good when it comes to putting up shelf advertising.
I don't know what Asda or Tesco do as it's ages since I was in either of those chains.
Some stores are not always up to date with their promotions and either don't have promotional material out or leave it up after the promotion has expired. If you pick up something which still has promotional material on display, even if the offer has expired, argue it - one of the managers will usually override the till and give you the promotion, then leg it to the shelf and remove the promotional material before it costs them too much.
The same thing can happen with shelf edge labels - new stock comes in with a higher price on the packaging, but they don't change the price on the shelf edge to match. Again, it's worth arguing it (you can always say you didn't notice the price on the packaging, unless it's blindingly obvious !) and you might get it at the lower price.
It's also worth checking multibuys on packaged items - I've seen a multibuy on packs of 40 tea bags which worked out more expensive than the 80 pack of the same brand before now (the larger pack was also on promotion, but just a price reduction and it wasn't as obvious as the multibuy).