So - you've decided to take the plunge and invest in a grow light. Next question: which type and why?
In order to answer this "how long is a piece of string" question, you first need to consider:
1) What do you want to use it for?
Different plants require different light intensities. Also, some growers want to use grow lights to supplement sunlight (e.g. in a greenhouse or conservatory) whereas others want to use growlights as a complete replacement for sunlight. Which general application area describes your intended usage best?
Do you intend to simply propagate seedlings or cuttings or do you wish to use your grow lights to power the full lifecycle? Seedlings require relatively little light compared to larger, more mature, light-loving plants entering the flowering and fruiting stage.
3) Light Levels
Do your plants tolerate low, medium or high light levels? For instance, cress, salad greens and herbs, and sprouts do not require very intense light at all in order to thrive-mushrooms even less. On the other hand heavy fruiters such as tomatoes and chili peppers or sun loving aromatic plants such as lavender and cilantro do extremely well in high light intensity conditions providing other facts such as temperature and relative humidity are kept in check.
4) Budget, Energy and Maintenance Costs
Grow lights can range massively in price, from as little as $30 (a T5 fluorescent propagation strip light) to between $1,000 and $7,000 for various fixtures based on LEP Plasma. In addition purchase price you should factor in the cost and frequency of lamp replacements and energy consumption too.
Once you have your answers to 1,2,3 and 4 - take a look at the different types of grow light available and see what fits.