Essentially you are asking about a cryptographic problem knows as "time lock encryption".
Obviously whatever time requirement you set can't be overcome by you (or anyone else) so it would only be useful for a "storage wallet". However you could have the private key be a SHA-256 hash CHAIN
(w/ a lot of iterations) of a memorized passphrase.
For example you make your passphrase "this is my password, no seriously stop hitting me with that wrench". Lets say you have a chip (GPU/CPU/FPGA/etc) that can perform 1 GH/s sequentially (note GPUs operate in parallel so sequential speed is much lower so you would need to do some benchmarking). Lets also say you KNOW you will not need access to the funds for at least a week.
1 * 10^9 * 60 * 60 * 24 * 7 = 2.52 x 10^13. Simply make the private key the SHA-256 hash of the hash of the hash of the hash .... 2.52 x10^13 iterations ... of the hash of your passphrase.
Spend the 1 week to produce the private key from the passphrase (but don't record it anywhere). Next generate the public address like normal. Now record the public address, passphrase, and # of iterations somewhere safe (like on plastic or metal card in a safe). Erase any copy of the private key (best done on say a linux live cd to ensure no copy remains).
Now you can deposit money at will by sending to the address but to remove funds requires knowing the passphrase AND performing 2.52 x 10^13 sequential SHA-256 hashes. Given the input of each hash is the output of the prior hash the problem can't be executed in parallel. The only thing that would speed things up is if the attacker has say a 2 GH/s (single chip not parallel execution) processor. Then it would take ~4 days instead of 7. 4GH/s would take ~2 days.
Even if attacker got a hold of say Deep bit = 3000 GH/s they couldn't speed up the solution because each hash is dependent on the one before. Deepbit is "fast" because Bitcoin hashing is only dependent on the prior block so all quadrillions of hashes of the current block are independent. Periodically (say every couple years) you should create a new address w/ more iterations and move the funds to ensure you stay ahead of Moore's law.
A chained hash is a very simple form of timelock encryption. The main drawback is to get 7 days in security requires 7 days to lock. There are more complex algorithms which allows you to use parallel resources to "lock" the secret but it can only be unlocked sequentially and that allows you to speed up the locking time without risking a faster unlocking.http://www.gwern.net/Self-decrypting%20files