Lamda is not what you want if you intend to have the app run for more than 5 minutes consecutively, or if it will need more than 1.5gb of memory. Lambda is designed for small, contained code snippets that don’t take long to run.
Ec2, or hosting your application on the beanstalk would allow it to run 24/7, or during a schedule you specify.
Most bots, if they are just an irc connection that monitors chat and responds to a single users channel are pretty safe to run from an ec2 nano or micro – you may need a beefier instance if you’re doing data storage and analytics, or providing web services - you won’t know until you measure resource requirement.
Things to just keep track of is your bandwidth that you send out, storage space requirements, cpu, and RAM. All of these items will cost you money if you go the ec2 or beanstalk route.
Some information about ec2 instance types, if you’re interested, https://aws.amazon.com/ec2/instance-types/