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http://www.skybrary.aero/bookshelf/books/2066.pdf
8che and slr737:
An alternate is an alternate and needs to be considered according to your applicable regulation. In EASA, this is governed by CAT.POL.230/235 for Perf A aircraft, which I presume slr737 is discussing. A takeoff alternate is no different.
There are many reasons why you may need to return after take-off, aside from OEI case. What weight would you plan for? An immediate return with a realistic circuit fuel burn would be sensible. Any more time aloft will only give you more favourable performance. If you have an unconfined fire or other such emergency, you may elect to return immediately regardless of the performance or structural limitations, should the situation dictate.
As with all of this, it's a planning criteria applicable up to the moment you begin the take-off run. After that, as safetypee points out, CAT.OP.MPA.300 applies. If you have planned things correctly during your preflight brief, you should have a good idea of where you're going in the event of certain triggers. i.e. a 'land as soon as possible' situation may warrant an overweight return to land; a 'land as soon as practicable' situation should warrant the take-off alternate which satisfied the dispatch requirements prior to take-off.
Consider your destination. Do you have to land within 60% of the LDA for a dry runway? No, you need to demonstrate that a realistic fuel burn will allow your theoretical landing performance to permit a landing within 60%. If you arrive at the destination heavier than planned (i.e. more favourable winds/flight level, direct routing, etc...), then what? You can still land, as long as the commander deems it's safe to do so prior to commencing the approach. Various companies have mandatory factors to help guide commanders with this decision. Factors are merely a guide, and no guarantee of achieving a safe landing as you can still royally balls up the TCH, Vref and flare.
LeadSled discusses the demonstrated landing performance. Your particular aircraft's configuration and the landing profile used to achieve this is in your AFM prior to the relevant performance. Read it. For the aircraft I've flown, they require maximum braking immediately after touching down and nailing the TCH and Vref speed, and glidepath angle. We generally don't do that. tends to spill the drinks, hence factoring is a good idea. Have a look at how an extra 10 knots on Vref affects your LDR. Or 10ft on TCH. All of this allows you to make an assessment of likely a safe landing can be achieved.