In Dota 2 some support items are strong if picked up early in the match, but they fall off quickly, and shouldn't be bought too late.
Most of the items are useful at all stages of the game.
A Glimmer Cape can save your life and the one of your carries in multiple situations. It is countered with a cheap Dust of Appearance, so the only difference between early and late game is an increased chance to find enemies with a Gem of True Sight.
There isn’t an exact rule on when it is ineffective buying an item, and often it also depends from the opponents you are facing, but generally you can use the following guidelines.
When is it too late for a Mekansm?
Many support players believe that the Mekansm is obligatory in every game, so they will try building it no matter what, even at 40+ minutes in the match. This is the biggest and most common mistake in pubs.
Mekansm isn’t only for supports
The item is great in the first 20 minutes of the match: it can revert the fate of a teamfight, and help during pushes and tower dives. The earlier you get it, the higher the chances to exploit its potential, so it should be an option on many cores that don’t need to rush damage items.
I suggest it on tanky mids and offlaners. It is a valuable pick on Bounty Hunter, who can easily build the Meka in the early game with some successful Tracks.
There is no exact maximum time for the Mekansm, but I discourage building it past 25 minutes.
Guardian Greaves are not a forced path after the Mekansm
The Greaves aren’t an early game item. Many players assume they can just build a Mekansm at any time because later on it is possible to upgrade it into the Guardian Greaves.
If your first item is a Mekansm at 35-40 minutes, you must accept that there are no chances to finish your build, and getting the Greaves.
Honestly, they are a very rare item. Even if your team is dominating, as a support is hard to save enough money to buy all your core and luxury items, and then to have also the urgency for an extra slot.
Don’t rush the Guardian Greaves after completing the Mekansm.
No Urn of Shadows past the 20 minute mark
No explanation is required. The Urn of Shadows is only an early game item, and later on you will sell it to free a slot.
For supports and gankers who roam a lot, the Urn is an incredible item because it will help to inflict extra damage or to heal the team before/after a battle.
Buy the Urn quickly if you plan to roam.
The Pipe of Insight is always useful
No time limit for the Pipe of Insight. If the team needs one, just buy it as soon as you can. Even in the late game it will be handy against heroes with high area of effect damage, such as Zeus or Ancient Apparition.
When a Pipe is desired, position 3 and 4 heroes should get it asap.
Drum of Endurance and Crimson Guard are very situational
The Crimson Guard isn't an item for supports: too expensive. If you are using a hero with a Vanguard, like Axe, consider upgrading it only when fighting heroes able to inflict multiple small instances of physical damage.
Phantom Lancer with his illusions, Broodmother with the Spiderlings, and Visage with the Familiars, are some of the best examples on when a Crimson Guard can make the difference between life and death for your teammates.
The Drum of Endurance was a more popular pick in the past, but today I don’t really suggest it anymore. It should be bought by the few heroes that synergize with it, like Spirit Breaker.
Crimson and Drum are for/against a defined group of heroes.
Other supportive items, such as the Force Staff, are always viable and shouldn't be restricted: they solely depends on your hero, team, and the opponents you are fighting.
This is all, just a final note: It is never too late to buy Wards :D!
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