It’s up to you if you want to dedicate a week, two or maybe even a year or more to working in voluntary programmes as there are many different opportunities and these can be fitted into your life at any time. If this is your first volunteering experience, you could consider a short-time programme, so that you could replace your traditional or all-inclusive holiday with this volunteering opportunity.
The length of the placement should also be considered with regards to your aims and the aims of the programme itself. Usually the longer the volunteering programme, the better the results for the community, organisation and the volunteer themselves.
You should also consider the climate of the places you are willing to go to as the climate and the seasons can impact on how comfortable you will feel - for example try to avoid the rainy season or monsoon season (unless for some reason this season is important for the programme you are likely to choose).
The weather is also a particularly important consideration when you want to combine volunteering with an additional holiday.