Having tried a lot of homemade pest control remedies, I'm thinking of using pesticides for a change but im not so sure on how to use it correctly. The label says to use 25ml per liter of water but nothing else and i was wondering if there's such a thing as "the best/worst time to apply it" so if anyone has tried this stuff, let me know on how to go about it.
Insecticides are best applied during the day (when you can see what you're doing), but preferably when the plant is not in direct sunlight, and preferably at dusk, when the bees and other pollinating insects have stopped working, although that is more important when plants are actually flowering - if they're not, you're less likely to spray any beneficial insects because they won't be there. Always spray beneath the leaves as well as the tops, and spray till run off... and avoid spraying flowers if possible. You don't say what type of insecticide you're using, but note that systemic insecticides work for up to two weeks, contact insecticides only kill what's on the plant when you spray, they confer no protection after that. If the container doesn't say what type it is, the recommendations for how often to apply will demonstrate which one it is - systemics usually say apply every 10-14 days (if necessary) or similar.
It depends on the insecticide.
First rule: read the label, but in your case you have no restrictions. Some insecticides should not be used when too hot, but some other not when it is too cold.
Second rule: if it is a contact insecticide (not for night pest), better to do in the early morning, when insects will sleep or not move much. I think night is also good.
Then you should check what pest you are fighting. From your comment on @Bamboo answer, it seems that you are thinking that good insect are active only during the day. This is not true (there are various flowers [especially white or very pale] which are pollinated only during night, not frequent on gardens, because you do not want flowers that you never see). But also some pests works only during day. Caterpillars often eat during night and hide during day. On the other hand we like many butterflies (but not moth, which are nocturnal).
Many insecticides do no arm bees (but some yes), so I would also check not to spray them when you see bees.
But weather is also important, you do not want to spray too early, if there will be dew in the morning. On the other hand, if you have less wind during night, night will be ideal (compared windy mornings or evenings, but depends on your locale climate).
Personally, I think day is better, because you can check better what to do, and so being more effective (all surfaces, and more regularly). On the other hand, you may use them also on night.
If it is very hot, night is better, on contact insecticides: you need to spray all surface, so very tiny drops, but they could evaporate before touching leaves. On agriculture, there are many studies about size of drops, speed of spraying and effectiveness (all depending on culture, pesticide, and climate), but this is mostly for professionals.