As explained by Joe, "You can never be sure that your opportunity won't be affected." To at least put out the appropriate feelers so that you can formulate your own decision, I suggest the following:
- Get an idea of the company's planned starting date
- Ask about whether the position is new or if you're filling someone's shoes
- Cover the request as part of the negotiations, not after accepting it
Ask about the starting date
In the earliest interview possible (phone interview or otherwise), you should have a chance to ask questions. I would recommend asking directly about the starting date:
"What starting date are you thinking of for this position?"
They may answer a specific date (e.g. "If hired, you would be expected to start on May 1st"), or a semi-concrete time period, (e.g. "We would look to have you start from mid-April to mid-May"), or they may turn the question around entirely, (e.g. "It depends on when you could start"). Getting an idea of what they are thinking will give you an idea of how far their expectations and your desires are apart.
Ask what type of position it is
There are primarily three types of positions:
- Totally new role
- Additional member in an existing team
- Replacing a member in an existing team
If the role is totally new (say a company creating a marketing role for handling social media), then there likely aren't any existing responsibilities, and they are more likely to be interested in hiring the right person over hiring a competent person now.
If you are being added to an existing team, it is likely because there is a lack of manpower at the moment. While they may be handling the load as-is for now, they are looking for a new person for a reason. This could go either way depending on the role.
If you are replacing someone who is leaving, then there are real constraints on the company-side because that person will also have their notice period, and will be leaving on a fixed day regardless of when you start. This has the least flexibility in regards to starting dates.
The biggest issue with asking this directly is that if the person is leaving for less-than-pleasant reasons (they were fired, moving to a competitor, etc.), you probably don't want that tainting your interviews in the earlier stages. You may be able to get some useful information by asking less direct questions (e.g. "How many people are working in that team?", etc.).
Start date is part of the negotiation
If you are in the final stage of interviews and think they will offer you the job, it's time to bring this up as part of negotiations. Job offers usually have a starting date as part of the offer, so like salary, it's best to bring it up before getting the offer if you want to negotiate it.
If this is an absolute must for you, then state it as such. If you are more concerned about keeping your options open, I'd start by bringing up the potential for an extended notice period at your current employer to see their reaction:
I have some unique skills and knowledge to transfer to my successors, my current employer may request that my notice period be extended up to two months to allow for a seamless transition.
If they react poorly, you can always say, "Don't worry, my contractual notice period is not two months, so this is negotiable." If they don't bat an eyelash and ask you when you want to negotiate your start date to be, then you can bring up the leave request. If they seem to be unwilling to go over two months but are okay with that period, you can negotiate a shorter notice period with your former employer to give you time off within those two months.